Chemistry of Superheavy Elements.

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Reviews
M. Schdel
DOI: 10.1002/anie.200461072
Superheavy Elements
Chemistry of Superheavy Elements
Matthias Schdel*
Keywords:
atom-at-a-time chemistry · periodic
table · relativistic effects ·
superheavy elements ·
transactinides
Dedicated to Professor Gnter Herrmann
on the occasion of his 80th birthday
Angewandte
Chemie
368
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 368 – 401
Angewandte
Chemie
Superheavy Elements
The number of chemical elements has increased considerably in the
last few decades. Most excitingly, these heaviest, man-made elements at
the far-end of the Periodic Table are located in the area of the longawaited superheavy elements. While physical techniques currently play
a leading role in these discoveries, the chemistry of superheavy
elements is now beginning to be developed. Advanced and very
sensitive techniques allow the chemical properties of these elusive
elements to be probed. Often, less than ten short-lived atoms, chemically separated one-atom-at-a-time, provide crucial information on
basic chemical properties. These results place the architecture of the
far-end of the Periodic Table on the test bench and probe the
increasingly strong relativistic effects that influence the chemical
properties there. This review is focused mainly on the experimental
work on superheavy element chemistry. It contains a short contribution on relativistic theory, and some important historical and nuclear
aspects.
1. Introduction and Historical Remarks
How many chemical elements do we know? How many
elements are sufficiently chemically characterized to justify
their position in the Periodic Table? Simple questions at every
chemist should be able to answer. But do you—do we—really
know?
The race for new elements beyond uranium started in the
mid-1930s involving groups in Rome, Berlin, and Paris.
Among the mistakes which led these scientists astray, were
presumptions about the structure of the Periodic Table at its
far end—the transuranium elements were assumed to belong
to Group 7 and the following Groups. The unexpected
discovery of nuclear fission[1] marked the first obstacle, and,
at the same time, brought new insight and opportunities[2, 3] .
Soon after, the first transuranium elements, neptunium and
plutonium were synthesized. The road to the discovery of
heavier elements, successfully applied in the synthesis and
separation of americium and curium, was opened when
Seaborg introduced the actinide concept.[4] This drastically
revised the Periodic Table (see ref. [5, 6] for an account of this
development, and ref. [7] for a detailed summary of the
chemistry of the actinides, thorium through lawrencium—
elements with atomic numbers Z = 90–103—which follow
actinium in the “actinide series”, and ref. [8] for a complete
coverage of the chemistry of transactinide elements).
The idea of the existence of chemical elements much
heavier than uranium emerged very early, at first as illusionary dreams in science-fiction literature. It was not until
the mid-1950s—when much was learned about the atomic
nucleus from investigations of its decay especially its fission
properties—that a scientifically sound discussion of the
possible existence of nuclei dubbed “superheavy” began
with contributions by John Wheeler[9] and Gertrude ScharffGoldhaber.[10] After the early success of treating the atomic
nucleus as a charged liquid drop (liquid-drop model) in
describing the nuclear fission process[11] a new quality appears
Angew. Chem. Int. Ed. 2006, 45, 368 – 401
From the Contents
1. Introduction and Historical
Remarks
369
2. Nuclear Aspects
372
3. Atom-at-a-Time Chemistry
374
4. Objectives for Superheavy
Element Chemistry
375
5. Experimental Techniques
376
6. Chemical Properties
380
7. Summary and Perspectives
394
with the quantized treatment of individual nucleons—protons and neutrons—in nuclear shell models. Similar to electrons in atoms
and molecules, and based on the same quantum mechanical
law, protons and neutrons form closed shells with “magic
numbers”, for example, 2, 8, 20, 28, 50, and 82. As with atoms
having closed electron shells, nuclei with closed shells exhibit
an extra and sometimes very pronounced stability (see
ref. [12] and references therein for a concise discussion of
the liquid-drop model and the shell contributions).
In the mid-1960s, this nuclear-shell theory received a large
boost from computer calculations based upon these new
theoretical understandings of the atomic nucleus. Until 1965 it
was conceivable that superheavy elements may exist around
Z = 126 (see Myers and Swiatecki>s calculations of nuclear
masses and deformations, ref. [13]). However, from then on,
new results focused on the Z = 114 nucleus with a neutron
number of N = 184 as the center of an “island of stability”.
Contributions came from Sobiczewski and co-workers[14] and,
during a conference at Lysekil[15] in 1966, from Meldner[16] and
others.[15]). First estimates[17–22] yielded relatively long halflives—as long as a billion years! These times encouraged the
search for superheavy elements (SHE) and their investigation
with chemical techniques. Among experimentalists, the hunt
started with searches for superheavy elements both in nature
and at accelerators (see refs. [12, 23–28] for reviews of this
early phase work).
At about the same time, the first Dirac–Fock and Dirac–
Fock–Slater calculations were performed for atoms to determine the electronic structure of superheavy elements.[29–34]
These results are summarized in ref. [35] They show that
extrapolating chemical properties along groups of elements in
[*] Dr. M. Sch'del
KPII–Kernchemie
Gesellschaft f,r Schwerionenforschung mbH
Planckstrasse 1, 64291 Darmstadt (Germany)
Fax: (+ 49) 6159-71-2903
E-mail: m.schaedel@gsi.de
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
369
Reviews
M. Schdel
the Periodic Table could be a valid approach for estimating
the chemical properties of superheavy elements. Simultaneously, the importance of a relativistic treatment of the
electronic orbitals was recognized. Several authors discussed
relativistic effects which might result in unexpected chemical
properties; see ref. [36–39] One of the articles was entitled
“Are elements 112, 114, and 118 relatively inert gases?”.[40] In
the last decade a breakthrough towards the theoretical
predictions of chemical properties was achieved with the
development of relativistic quantum molecular theories
applied for heavy and superheavy elements; reviews are
given in.[41–44]
Let us come back to the question, how many elements do
we know today? To answer this we have to be aware that the
“discovery” of an element 1) “is not always a single, simply
identifiable event or even culmination of a series of
researches … but may rather be the product of several
series of investigations … ”[45] and 2) that the judgment of
what is sufficient evidence to convince the scientific community that the formation of a new element has, indeed, been
established, may vary from group to group.[45] Because of
conflicting discovery claims and associated disputes over the
naming of the elements, a working group was jointly
established in 1986 by the International Union of Pure and
Applied Physics (IUPAP) and the International Union of
Pure and Applied Chemistry (IUPAC). At first, this Transfermium Working Group (TWG) established a set of criteria
that must be satisfied before the discovery of a new element is
recognized. Secondly, beginning with element 101, it evaluated all discovery claims until the year 1991.[45] This work was
continued by the IUPAC/IUPAP Joint Working Party (JWP).
Based on their recent report, the last “discovered” chemical
element[46] has atomic number 111; synthesized and identified
at the Gesellschaft fIr Schwerionenforschung (GSI) by
Hofmann et al.[47] in 1995 and recently substantiated[48] and
confirmed.[49, 50] Following a proposal by the discoverers, the
IUPAC has named element 111 roentgenium with the symbol
Rg[51] just one year after element 110 was baptized darmstadtium, Ds;[52] to honor the city of Darmstadt (Germany)
where the GSI is located. The official IUPAC Periodic Table
presently ends at element 111.
To assure credit for Hofmann et al.,[48, 53] for the discovery
of element 112—this experiment was again performed at the
recoil separator SHIP at GSI>s UNILAC accelerator—the
Matthias Schdel earned his PhD (1979)
from the Johannes Gutenberg University
Mainz. As a postdoc he worked at the
Lawrence Livermore and the Lawrence Berkeley National Laboratories with E. K. Hulet
and G. T. Seaborg. Since 1985 he has led
the nuclear chemistry group at GSI. He
organized and chaired the 1st International
Conference on the Chemistry and Physics of
the Transactinide Elements (1999). He is
the editor of the first comprehensive book on
the chemistry of superheavy elements. His
research interests focus on all nuclear and
chemical aspects of transactinides.
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JWP[46] has requested a confirmation experiment. The findings by the SHIP group were strongly supported by results
from the first chemical separation and investigation of
element 108 (this experiment will be discussed in detail in
the chemistry Section of this Review).[54] A direct confirmation of the production and the decay of the isotope 277112 was
obtained by Morita and co-workers[55] at The Institute of
Physical and Chemical Research (RIKEN) in Wako (Japan)
with the same technique as used for elements 110 and 111.[49]
With high confidence, we can anticipate that the discovery of
element 112 will be accepted soon and that the assigned
priority for the discovery will go to the SHIP group. Reviews
of this group>s work, including the discoveries of element 107
(bohrium, Bh), element 108 (hassium, Hs), and element 109
(meitnerium, Mt) can be found in ref. [56–62]
A world-record low cross-section—and therefore
extremely difficult to repeat and to confirm—was reached
by Morita and co-workers in their recently reported finding of
one atom of element 113.[63] All the above mentioned nuclides
are the ones in the upper-left part of Figure 1, which shows the
uppermost part of the chart of nuclides. From a chemist>s
point of view, an important characteristic feature of the
nuclides produced in nuclear reactions with Pb and Bi targets
yields only short-lived products with millisecond half-lives.
This life-time prohibits chemical studies with virtually all of
the presently available techniques. However, new technological developments will also allow, to some extent, to exploit
nuclides produced from some types of nuclear reactions for
chemical investigations.
But there are even more chemical elements—and longer
lived isotopes of known elements—on the horizon and these
are especially exciting for chemists. Oganessian et al. have
performed an extended series of experiments irradiating
actinide targets with 48Ca at the Flerov Laboratory of Nuclear
Reactions (FLNR) in Dubna (Russia) to produce even
heavier elements—and more neutron-rich, longer-lived isotopes of known elements (see upper-right part of Figure 1 and
Section 2 for more details of the nuclear reactions used and
the decays observed). The discoveries of elements 113–116,
and the weak evidence for element 118, (see refs. [64–66]) are
currently waiting to be confirmed. In producing nuclei close
to the former “island of stability” around Z = 114 and N =
184, these experiments suffer a disadvantage in that their
nuclear decay is not “genetically” linked by unequivocal a–a
decay sequences to the region of known nuclei—a prerequisite used by the SHIP group for the unique identification.
Chemistry—in addition to unraveling exciting chemical
properties of these elements—may become a crucial tool in
elemental identification. The first steps towards a chemical
separation and identification of element 112 have been
made[67–69] and, as this is one of the currently hottest topics
in nuclear chemistry, more experiments are under way.[70] The
way was paved and the run started with the report[71] of the
first observation of the nuclide 283112 at the recoil separator
VASSILISSA in Dubna. The nuclide 283112, produced in a
reaction with a 48Ca beam and 238U as a target, supposedly has
a half-life (t1/2) of about a minute—long enough to perform
chemical separations with single atoms. More recent reports
began to revise the decay properties[66] of this nuclide,
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chemie
Superheavy Elements
Figure 1. Upper part of the chart of nuclides. Half-lives and color-coded nuclear decay modes (yellow = a-decay, green = spontaneous fission,
red = electron capture; see also Section 2) are given together with the mass number for each nuclide. Regions of enhanced nuclear stability
around Z = 108, N = 162 (dashed line) for deformed nuclei and around Z = 114 (solid line), N = 184 (outside the drawn area) for spherical nuclei
are indicated in dark blue. Adapted from ref. [62] with the shell-stability calculations of Sobiczewski and co-workers.
however, it still has a t1/2 in an accessible region for chemical
studies.
Now, we turn back to chemistry. Element 104, rutherfordium, Rf, marks the beginning of a remarkable series of
chemical elements: From a nuclear point of view, they can be
called superheavy elements—as they only exist because of
their microscopic shell stabilization (see Section 2 for a
detailed discussion of this aspect)—and from a chemical point
of view they are transactinide elements—because the series of
actinides[4] ends with element 103. One of the most important
and most interesting questions for a chemist is that of the
position of SHE in the Periodic Table of the Elements and
their related chemical properties—especially in comparison
with the lighter homologues in the respective groups
(Figure 2).
From atomic calculations[32, 41, 43, 44] , it is expected that the
filling of the 6d electron shell coincides with the beginning of
the series of transactinide elements. Consequently, chemical
behavior similar to that known from the transition metals in
the fifth and sixth periods is anticipated. However, it is by no
means trivial to assume that rutherfordium in Group 4 of the
Periodic Table—and the heavier elements in the following
groups—will exhibit chemical properties, which can in detail
easily be deduced from their position. To which extent the
Angew. Chem. Int. Ed. 2006, 45, 368 – 401
Figure 2. Periodic Table of the Elements. The known transactinide
elements 104–112 should take the positions of the seventh-period
transition metals below Hf in Group 4 and Hg in Group 12. chemical
studies have placed the elements Rf–Hs into Group 4–8. The “chemically unknown” heavier elements (full symbols for known elements
and open symbols for as yet unconfirmed reports) still need to be
investigated. The arrangement of the actinides reflects that the first
actinide elements still resemble, to a decreasing extent, the chemistry
of d-block elements: Th below Group 4 elements Zr and Hf, Pa below
Nb and Ta, and U below the Group 6 elements Mo and W.
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M. Schdel
Periodic Table is still a valid ordering scheme regarding
chemical properties of the SHE is one of the key questions.
Modern relativistic atomic and molecular calculations[41–44] clearly show the very large influence of direct and
indirect relativistic effects on the energetic position and the
sequence of electrons in their respective orbitals. These
effects are also associated with changes in their radial
distributions. All of these relativistic changes are so pronounced compared to the results of non-relativistic calculations, that it would not be surprising if the SHE had
significantly different chemical properties to those anticipated. Therefore, it is of great interest to study chemical
properties of SHE in detail and to compare these with the
properties deduced from extrapolations and from modern
relativistic molecular calculations in combination with empirical models. First-generation experiments with rutherfordium[72–74] and element 105,[75, 76] dubnium, Db, gave enough
justification to place Rf into Group 4 and Db into Group 5 of
the Periodic Table. Chemical properties of SHE, or transactinide elements, have been studied up to element 108 (see
ref. [8] for a complete compilation) and the first experiments
are under way to reach element 112 and beyond.
This Review briefly deals, in its first part, with important
nuclear aspects related to the synthesis and nuclear decay of
superheavy elements—including a definition of SHE. It will
be shown that only single, short-lived atoms are available for
these kinds of chemical studies. This section is followed by a
short discussion of recent theoretical work including predictions of chemical properties. The main part (Sections 5 and 6)
focuses on 1) experimental techniques, 2) some key experiments to unravel detailed chemical properties of elements 104
and 105 in the liquid phase and in the gas phase, 3) first survey
experiments of element 106, seaborgium, Sg, and 4) the first,
successful experiments on element 107, bohrium, Bh, and on
element 108, hassium, Hs, performed in the gas phase. For a
complete coverage of this field see ref. [8] Comprehensive
reviews can be found in refs. [77–81] To finish this Introduction it may be appropriate to quote Friedlander and
Herrmann stating “… the upward extension of the Periodic
Table … has been one of the triumphs of nuclear chemistry in
recent decades”.[82]
and that sandbanks and rocky footpaths connect the region of
shell-stabilized spherical nuclei to our known world. In
addition, recent theoretical results indicate that the atomic
numbers 126[87] and, more likely, 120[88]) are also closed shells;
with possibly even more pronounced shell stabilization than
for element 114.
Perfectly acceptably, some authors are still using the term
SHE in connection with spherical nuclei only. However,
others have widened this region and have included lighter
elements as, for example, already discussed in an article by
Sobiczewski, Patyk, and Cwiok entitled “Do the superheavy
nuclei really form an island?”.[89] An argument is developed[90]
to show that it may be well justified to begin the superheavy
elements with element 104. The result is especially appealing
in as much as the beginning of superheavy elements coincides
with the beginning of the transactinide elements.
Two definitions or assumptions are used: 1) Superheavy
elements is a synonym for elements which only exist due to
their nuclear-shell effect. 2) Following arguments given in
ref. [45, 91] only those composite nuclear systems that live at
least 1014 s shall be considered a chemical element. This time
is well justified from nuclear aspects, for example, from
maximum lifetimes of excited compound nuclei (see Section 2.2), as well as from chemical aspects, for example, from
the minimum formation time of a molecule such as hydrogen.
We now apply these two assumptions to a comparison of the
calculated and the experimentally observed spontaneous
fission half-lives—the most drastic, spontaneous disintegration process of a very heavy nucleus. The results are shown in
Figure 3 plotted against a frequently used (in nuclear physics)
2. Nuclear Aspects
2.1. The Region of Superheavy Elements
Characteristic electronic and chemical properties allow
the beginning of the transactinide elements to be placed at
element 104—but where do the SHE begin? Until the early
1980s a straight forward answer would have pointed towards
the remote “island of stability” centered at Z = 114 and N =
184 which was surrounded by a “sea of instability”.[12, 83] Up to
that time, and typical for closed-shell nuclei, SHE were
expected to have a spherical shape. However, based upon
more recent experimental results[57, 59, 84] and theoretical concepts, which take into account shell-stabilized deformed
nuclei and emphasize the importance of the N = 162 neutron
shell,[85, 86] we know that the sea of instability has drained
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Figure 3. Known spontaneous fission (sf) half-lives (t1/2) of nuclides
with even numbers of protons and neutrons (dots) and calculated
hypothetical half-lives (dashed line) taking into account only the liquiddrop-model contribution plotted versus the fissility parameter X. The
dotted line shows the lifetime-limit of 1014 s for a chemical element.
From ref. [90].
fissility parameter, X.[92] This parameter goes with Z2/A (Z is
the atomic number of the nucleus and A is its mass) and it
takes into account the proton-to-neutron ratio in a nucleus.
This parameter reflects the increasing tendency to spontaneous fission in progressing to heavier nuclei.
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Superheavy Elements
As only the macroscopic liquid-drop part of the nucleus
was taken into account in calculating these half-lives (dashed
line in Figure 3), the difference between experimentally
known half-lives and calculated values reflects the additional
shell stabilization of the nuclei[56] . It can be seen from Figure 3
that at a fissility parameter of 0.88—located between
nobelium and rutherfordium—the hypothetical “liquid-drop
half-lives” drop below the 1014 s margin while the shell
contribution allows these nuclides to have lifetimes up to
factors of about 1015 longer. From this it can be claimed that
all elements beginning with element 104—the transactinides—live only because of their microscopic shell stabilization and, therefore, should be called superheavy elements.
2.2. Nuclear Syntheses
While transuranium elements up to and including ferFigure 4. Pictorial view of the 208Pb(58Fe,1n)265Hs reaction as an
mium (Z = 100) can be produced by stepwise neutron capture
example for a cold-fusion reaction and 248Cm(26Mg,4–5n)269–270Hs for a
and subsequent b-decay in a high (neutron) flux nuclear
hot-fusion reaction.
reactor, transfermium elements can only be man-made by
nuclear fusion reactions with heavy ions in accelerators.[5, 6, 60]
three or four more neutrons, these reactions are applied to
In the accelerator-based reactions, the Coulomb barrier
synthesize the most neutron-rich and relatively long-lived
between the two approaching positively charged, atomic
isotopes used in chemical investigations of SHE. Half-lives,
nuclei always has to be overcome. Therefore, the combined,
method of the syntheses, and cross sections are summarized in
fused system, which is called the compound nucleus, always
Table 1 (from ref. [90, 94]). More detailed discussions about
carries a certain amount of excitation energy. The availability
specific aspects of hot-fusion reactions can be found in
of suitable ion beams and target materials—and the energy
ref. [57, 60, 95, 96]
balances associated with these combinations—allow a crude
distinction between two types of reactions: One frequently
termed “cold fusion” and the other one “hot
fusion”.
Table 1: Nuclides from hot-fusion reactions (and the cold-fusion reaction Ti + Pb) used in SHE
Cold-fusion reactions are characterized
chemistry.[a]
by relatively low nuclear excitation energies
Nuclide
t1/2 [s]
Target
Beam
Evap[b]
s[b]
v(c)
of about 10–15 MeV. They occur when
261m
248
18
Rf
78
Cm[d]
O
5
10 nb
3 min1
medium-heavy projectiles, for example,
22
244
58
Pu[e]
Ne
5
4 nb
1 min1
Fe, 62,64Ni, or 68,70Zn, fuse (at the lowest
208
[e]
50
257
208
209
Rf
4
Pb
Ti
1
15
nb
5
min1
possible energy) with Pb or Bi target
249
18
262
Db
34
Bk[d]
O
5
6 nb
2 min1
nuclei. There are many advantages with this
19
248
Cm[d]
F
5
1 nb
0.3 min1
reaction which, among others, helped to
249
18
263
Db
27
Bk[d]
O
4
10 nb
3 min1
249 [e]
18
263
discover elements 107–112.[57, 59, 60] However,
Sg
0.9
Cf
O
4
300 pb
6 h1
248
22
265
a severe disadvantage for chemical studies is
Sg
7.4
Cm[d]
Ne
5
240 pb
5 h1
248
[d]
22
266
Sg
21
Cm
Ne
4
25
pb
0.5
h1
the very short half-lives of the relatively
249
[d]
22
267
Bh
17
Bk
Ne
4
70
pb
1.5
h1
neutron-deficient nuclei produced. An illus248
26
269
Hs
14
Cm[d]
Mg
5
6 pb
3 d1
trative view of this reaction mechanism is
248
26
270
Hs
2–7
Cm[d]
Mg
4
4 pb
2 d1
given, for example, in ref. [12] and in
[a] Data from ref. [90, 94]. [b] s = cross section, Evap = number of emitted neutrons. [c] Production rate
Figure 4. Except for one specific type of
assuming typical values of 0.8 mg cm2 for the target thickness and beam intensities of 3 J 1012 particles
experiment[93] cold-fusion reactions are usuper second. [d] Reaction commonly used in chemistry experiments. [e] Reaction rarely used or only in
ally not used in chemical studies of the
very specific experiments, or the nuclide is only observed as a by-product.
heaviest elements.
Hot-fusion reactions are characterized
For cold-fusion reactions, and hot-fusion reactions, the
by excitation energies of about 40–50 MeV
cross sections—the probability of forming the desired prodwhen actinide target nuclei, such as 238U, 242,244Pu, 243Am,
248
uct—constantly decrease with increasing atomic number of
Cm, 249Bk, 249Cf, and 254Es, fuse with light-ion beams, such as
18
the product. In cold-fusion reactions, decreasing cross secO, 22Ne, and 26Mg (see Figure 4). Whereas in cold-fusion
tions are presumably due to an increasing fusion hindrance of
reactions usually only one neutron is evaporated, four or five
the highly charged nuclei (Ztarget O Zproj.). In hot-fusion reacneutrons are emitted in hot-fusion reactions before the
compound nucleus has cooled. Because of the neutrontions it is predominantly the strong fission competition
richness of the actinides targets, and despite the emission of
(fission versus neutron evaporation) in the deexcitation
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M. Schdel
process of the hot compound nucleus which diminishes the
cross section.
Nuclear reaction cross sections (s) are measured in barn
(b); 1 b = 1024 cm2. This number is related to simple geometric arguments concerning a projectile hitting a target
nucleus. A “typical” nucleus has a radius of about 6 O 1013 cm
(= 6 fm, femtometer); for example, the nuclear radius (r) of
Zn is 4.9 fm and of Pb is 7.1 fm.[61] As the geometric cross
section of a nucleus is pr2, a value of about 1024 cm2 results
for a “typical” nucleus. While some nuclear reactions have
cross sections between several barn and millibarn, heavy
actinides are usually produced with microbarn. Cross sections
for the syntheses of n-rich, transactinides in hot-fusion
reactions vary from about ten nanobarn (1 nb = 1033 cm2)
to a few picobarn (1 pb = 1036 cm2). The production rate is
the product of three terms: The cross section (in cm2), the
number of target atoms (in cm2), and the flux of projectiles
(usually in s1).
With typical beam intensities of 3 O 1012 heavy-ions per
second and targets of about 0.8 mg cm2 thickness (ca. 2 O
1018 atoms cm2), production yields range from a few atoms
per minute for Rf and Db isotopes to five atoms per hour for
265
Sg[97]), to some tens of atoms per day for 267Bh[98, 99] , and a
few atoms per day for 269Hs[54, 100] . Therefore, all chemical
separations are performed with single atoms on an “atom-ata-time” scale. An additional complication for the experimenter arises from the fact that the time at which the
synthesis of an individual atom occurs is unknown, as it is
produced in a statistical process.
As briefly sketched in Section 1 (see there for references)
experiments with actinide targets and a 48Ca beam give strong
evidence for the existence of relatively long-lived nuclides of
elements 112–116 and their a-decay daughter products (see
upper right part of Figure 1). Somewhat surprisingly, the
relatively high cross sections—a few picobarn, that is,
production rates of the order of about one atom per day—
seem to be almost constant in this region.[60, 65, 66] One
interpretation of this rather pleasant but not fully understood
effect sees the origin in the doubly magic character of 48Ca
with closed shells at Z = 20, N = 28. Reactions with 48Ca as a
projectile may gain some of advantages of two sides: 1) From
the cold-fusion reactions—magic nuclei (target or projectile)
allow for a formation of cold compound nuclei with low
fission competition—and 2) from the hot-fusion fusion reactions—larger asymmetry in the nuclear charge of the target
and projectile eases the fusion process. There is considerable
optimism that these reactions could extend chemical studies
into the region of element 114.
2.3. Nuclear Decay
Nuclear chemistry techniques are not only highly efficient
to collect products from nuclear reactions but are also well
adapted to half-lives of a few seconds and longer. Therefore, it
is not surprising that many of the longer-lived, neutron-rich
isotopes of the heaviest actinides and early transactinides
were discovered or were first studied applying these techniques. Alpha decay is the most characteristic decay mode in
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this region of the chart of nuclides (see Figure 1), and
provides a unique nuclide identification of the investigated
product. In particular, time-correlated consecutive mother–
daughter a–a-decay chains provide unambiguous signals.
They are used to identify these nuclides in specific chemical
fractions or at characteristic positions after chemical separation. The observation of an a-particle or a fragment from
spontaneous fission (sf) is the only means of detecting an
individual atom after chemical separation and this can be
performed with a very high efficiency. A number of nucleardecay properties were determined in the course of the
chemistry experiments as a “by-product” of these investigations. Specifically designed experiments using chemical
separation techniques are given in ref. [90] and references
therein. These experiments not only yielded new isotopes or
decay modes but were also instrumental in confirming[54] the
discovery of element 112.
3. Atom-at-a-Time Chemistry
The one-atom-at-a-time appearance of superheavy elements poses some unique problems for the chemistry at the
end of the Periodic Table. As a single atom cannot exist in
different chemical forms taking part in the chemical equilibrium at the same time, the classical law of mass action—well
established for macroscopic quantities and characterizing a
dynamic, reversible process in which reactants and products
are continuously transformed into each other—is no longer
valid.[101, 102] For single atoms, the concept of chemical
equilibrium needs to be substituted by an equivalent expression in which concentrations, activities, or partial pressures
are replaced by probabilities of finding the atom in one state
or the other. An atom can sample these states with
frequencies of hundreds (and more) exchange reactions per
second if the chemical system is selected such that the free
enthalpy of activation between these states is below 17 kcal
( 70 kJ).[103]
The time one atom (or molecule) spends in one state or
another—the measure of its probability of being in either
phase—can be determined in dynamic partition experiments.
These experiments are characterized by the flow of a mobile
phase relative to a stationary phase while a single atom is
frequently changing between the two phases. This situation is
realized in many chromatographic separations, for example,
in the exchange between a gaseous and a solid phase (wall
adsorption) in gas chromatography or between a mobile
liquid phase and a stationary ion-exchange resin in liquid
chromatography. In these processes, the retention or elution
times provide information about the average time an atom
has spent in either phase. Such characterizations of the
behavior of a single atom yield information which approximates the equilibrium constant that would be obtained from
macroscopic amounts of this element. More detailed information on this situation can be found in refs. [79, 81, 104]
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4. Objectives for Superheavy Element Chemistry
4.2. Relativistic Effects
4.1. Architecture of the Periodic Table
A detailed discussions of relativistic effects in general and
specifically for superheavy elements can be found in
ref. [38, 39, 106] and ref. [43, 44, 108], respectively. The relativistic increase in mass is known given by Equation (1) where
The Periodic Table of the Elements (see Figure 2 for one
possible version similar to the cover of ref. [105]) is the basic
ordering scheme for chemical elements and the most important and useful tool in predicting their chemical behavior.
Conceptually, it is, at first, the atomic number and the
associated electronic configuration of an element that define
its position in the Periodic Table. Secondly, related to this
position are chemical properties that arise from the electronic
configuration. Trends in the chemical behavior can be linked
to trends in the electronic configurations along groups or
periods in this scheme. However, as was painfully experienced
in the early searches for transuranium elements (see Section 1), simple extrapolations of existing periodic properties
must be used cautiously. This is especially true for superheavy
elements where relativistic effects on the electronic structure
become increasingly strong (see Section 4.2) and will significantly influence the properties of these elements. Deviations
from the periodicity of the chemical properties, based on
extrapolations from lighter homologues in the Periodic Table,
have been predicted for some time (see ref. [29, 32, 35, 40] and
references therein). In more general terms, the issue of
“Relativity and the Periodic System of Elements” has been in
the focus for some time.[38, 39, 106]
It is one of the highest priorities of the theoretical and the
experimental “heavy-element” chemists> work to predict—
and to validate or contradict—the chemical behavior of SHE,
especially in relation to their position in the Periodic Table.
Recently, a new wave of theoretical and experimental
investigations has led to a better understanding of the
chemistry of superheavy elements. Relativistic quantumchemical treatments, which reliably calculate the electronic
configurations of heavy-element atoms, ions, and molecules—
combined with fundamental physicochemical considerations
of the interactions of these species with their chemical
environment—now allow detailed predictions of the chemical
properties of superheavy elements. These properties are often
compared with empirical, linear extrapolations of the chemical properties found along groups and periods to disclose the
impact of relativistic effects. However, the empirical extrapolations are not purely non-relativistic, as relativistic effects
are, to some extent, already present in the lighter elements.
An additional complication for such assessments, is the
competition between relativistic and shell-structure effects.
This competition obscures a clear-cut correlation between an
observed chemical property and one specific effect. It poses
an additional challenge for a deeper understanding of the
chemistry of elements at the uppermost reaches of the
Periodic Table (and for the table>s architecture) especially if
purely empirical predictions are to be improved upon.
However, a number of landmark accomplishments resulted
from a number of new and detailed experimental findings and
theoretical results over the last decade. For comprehensive
summaries and reviews of the theoretical work see refs. [41–
44, 107, 108], for the experimental techniques see
refs. [79, 81, 90, 94, 109], and ref. [8] for a complete coverage.
Angew. Chem. Int. Ed. 2006, 45, 368 – 401
m ¼ m0 =½1ðv=cÞ2 1=2
ð1Þ
m0 is the electron rest mass, v is the velocity of the electron,
and c the speed of light. The effective Bohr radius [Eq. (2)]
decreases with increasing electron velocity.
aB ¼
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
2
h
¼ a0B 1ðv=cÞ2
m c2
ð2Þ
This orbital contraction and stabilization of the spherical s
and p1/2 electrons—the “direct relativistic effect”—was originally thought to be important only for the “fast”, inner K and
L shell electrons. However, it has been realized that the direct
relativistic effect is still large even for the outermost s and p1/2
valence electrons in superheavy elements. Thus, for example,
the 7s orbital electrons of element 105 are relativistically
contracted by 25 % and energetically stabilized.[43] Figure 5
Figure 5. Relativistic (c) and non-relativistic (a) radial
distribution of the 7 s valence electrons in element 105, Db.
(1 a.u. = 52.92 pm). Figure adapted from ref. [43].
shows the radial distribution of the “relativistic” 7 s valence
electron compared with a hypothetical “non-relativistic” one.
The second relativistic effect—the “indirect relativistic
effect”—is the expansion of outer d and f orbitals. The
relativistic contraction of the s and p1/2 shells results in a more
efficient screening of the nuclear charge, so that the outer
orbitals, which never come close to the core, become more
expanded and energetically destabilized. While the direct
relativistic effect originates in the immediate vicinity of the
nucleus, the indirect relativistic effect manifests itself in the
outer core shells. As an example, for Group 6 elements,
Figure 6 shows the stabilization of the ns orbitals, as well as
the destabilization of the (n1)d orbitals. The increasingly
strong influence of the relativistic effects on the absolute and
relative position of the valence orbitals can be seen. This
feature is most pronounced for element 106, Sg, where the
level sequence of 7s and 6d orbitals is inverted.
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Table 2: Ground-state electronic configuration and stable oxidation
states for elements 104–118.[a]
Figure 6. Relativistic (rel.; Dirac–Fock calculation) and non-relativistic
(nr.; Hartree–Fock calculation) energy levels of the Group 6 valence ns
and (nl)d electrons. Figure adapted from ref. [108] with data from
ref. [33].
A non-relativistic description—calculation or empirical
extrapolation within Group 6—would result in a much different and incorrect description of the electronic level configuration for seaborgium. It can be anticipated that these
drastic changes may lead to unusual oxidation states, ionic
radii that are very different to those predicted from simple
extrapolations in a specific group, or significant changes in the
ionic and the covalent portions of a chemical bond.
The third relativistic effect is the “spin-orbit (SO)
splitting” of levels with l > 0 (p, d, f,… electrons) into j = l 1
=2 states. This effect also originates in the vicinity of the
nucleus. For orbitals with the same l value, the SO splitting
decreases with increasing number of subshells, that is, it is
much stronger for inner shells than outer shells. For orbitals
with the same principal quantum number, the SO splitting
decreases with increasing l value. In transactinide compounds
the SO coupling becomes similar, or even larger, in size than
typical bond energies. The SO splitting of the valence 7p
electrons in element 118, for example, may be as large as
11.8 eV.[43]
Each of the three effects (direct and indirect relativistic
effect and SO splitting) is of the same order of magnitude and
grows roughly as Z2 ! This is one of the reasons why it is most
fascinating to experimentally probe the highest Z elements.
Other effects, such as the Breit effect (accounting for
magnetostatic interactions) and the QED effect (vacuum
polarization and self-energy) are not negligible but of minor
importance for chemical properties of SHE.[108]
4.3. Atomic Properties
It is helpful to remember that not only the electronic
ground-state configurations (see Table 2) but also other
properties, such as ionization potentials, atomic/ionic radii,
and polarizabilities, are important parameters which eventually determine the chemical behavior of an element.
A detailed discussion of the theoretical determination of
these parameters, and their influence on the chemical
behavior of SHE, is given by Pershina in ref. [43, 44] and
references therein. Knowing the trends in these properties
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Element
Group
Electronic
configuration
(core: [Rn]5f14)
Stable
oxidation
state[b–d]
104, Rf
105, Db
106, Sg
107, Bh
108, Hs
109, Mt
110, Ds
111, Rg
112
113
114
115
116
117
118
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
6d27s2
6d37s2
6d47s2
6d57s2
6d67s2
6d77s2
6d87s2
6d97s2
6d107s2
6d107s2
6d107s2
6d107s2
6d107s2
6d107s2
6d107s2
3, 4
3, 4, 5
4, 6
3, 4, 5, 7
3, 4, 6, 8
1, 3, 6
0, 2, 4, 6
1, 3, 5
0, 2, 4
1, 3
0, 2, 4
1, 3
2, 4
1, 1, 3, 5
2, 4, 6
7p
7p2
7p3
7p4
7p5
7p6
[a] Data from ref. [43, 44]. [b] Bold = most stable oxidation state in the
gas phase. [c] Underlined = most stable in aqueous solution if different
from gas phase. [d] Italics = experimentally observed oxidation states.
from theoretical calculations helps to assess similarities (or
differences) of SHE properties in relation to the properties of
their lighter homologues in the Periodic Table. Even if it
would be premature (or sometimes even misleading) to judge
the chemical properties purely from the electronic groundstate configurations given in Table 2—together with the most
stable oxidation states—they can provide some guidance.[43]
Earlier predictions of chemical properties are summarized in
ref. [35].
5. Experimental Techniques
Fast chemical-isolation procedures to study the chemical
and physical properties of short-lived radioactive nuclides
have a long tradition and have been used since the beginning
of radiochemistry. The rapid development of increasingly fast
and automated chemical-separation techniques originated
from the desire to study short-lived nuclides from nuclear
fission (see ref. [110, 111] for reviews). Also the discovery of
new elements up to Md (Z = 101) was accomplished by
chemical means.[5] Although from there on, physical techniques prevailed in the discoveries, rapid gas-phase separation
chemistry played an important role in the discovery claims of
elements 104 and 105.[45] Today, the fastest chemical-separation systems allow the study of the nuclides of transactinide
elements with half-lives of less than 10 s. Reviews on these
methods and techniques with varying emphasis can be found
in ref. [77–79, 112–115] and a recent and comprehensive
coverage in ref. [116]
Experiments can be grouped into the following steps:
1) Synthesis of the element.
2) Rapid transport of the synthesized nuclide to the chemical
apparatus.
3) Formation of a desired chemical species or compound
(this can be done before, during, or after the transport).
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4) Fast chemical separation and chemical characterization.
5) Preparation of a sample suitable for nuclear spectroscopy.
6) Detection of the nuclide through its characteristic nucleardecay properties.
Flow-schemes for such online experiments with transactinides are shown in Figure 7. The atom-at-a-time nature of
SHE chemistry requires stringent optimizations of all of these
identifying a single atom after chemical separation. Thus
at the end, or after a separation process, samples must to
be prepared that are suitable for analysis by, for example,
high-resolution a-spectroscopy.
6) As the type of chemical species cannot be determined
during or after the transactinide separation, the chemical
system must be chosen such that a certain chemical state is
probable and stabilized by the chemical environment.
Several approaches have been successful in studying the chemical properties
of superheavy elements. One of the main
distinctions between the different
approaches, is that one type of experiment
works in the liquid phase and the other in
the gas phase. The same distinctions are
made in the following subsections, but
first the common initial parts of the
experiment are discussed.
5.1. Beams, Targets, Collection, and
Transport
Heavy-ion beams, such as 18O, 22Ne,
Mg, and 48Ca—with velocities of about
10 % the speed of light and typical intensities of 3–6 O 1012 particles per second—
are
delivered from an accelerator to the
Figure 7. Flow-scheme for two types of online chemistry experiment. Left: Transport of
nuclear reaction products with aerosols (cluster) and formation of a chemical compound in
experiment. There, they pass first through
the chemistry apparatus; typical for the chemistry of elements Rf to Bh. Right: Transport of
a vacuum isolation window and a targetvolatile species (atoms or compounds formed in the recoil chamber) to the chemistry set up
backing before interacting with the acti(which is sometimes in one unit together with the detectors); typical, for Hs and element 112
nide target material. The energy loss of
chemistry.
the projectiles creates heat which must be
removed to prevent damage to the
window and the target. For this purpose, wheels with rotating
steps. The chemical-separation system has to fulfill several
windows and targets as well as stationary arrangements with
prerequisites simultaneously.[116]
double-windows and forced gas cooling are used.[116–118]
1) Speed becomes increasingly important from the lighter
261
elements, such as Rf (t1/2( Rf) = 78 s), to the heavier ones,
A stationary apparatus is schematically shown in
such as Hs (t1/2(269Hs) 14 s).[48]
Figure 8.[118] Cooling gas is forced at high velocity through a
narrow gap between the 6 mm diameter vacuum isolation
2) A sufficiently high number of exchange steps are required
window and the target backing.[117] Typical target thicknesses
for an individual atom or compound to ensure that its
behavior is characteristic of the element.
are about 0.8 mg cm2. Mainly electrodeposition and molec3) The system needs to be selective enough, not only to
ular plating methods have been used in recent years to deposit
probe a specific chemical property, but also to separate
the target material onto the backing. The advantages and
other unwanted nuclear reaction by-products which may
limitations of these techniques are discussed in ref. [116, 117]
obscure a unique identification of the atom under invesTo allow increased beam intensities beyond the limit of
tigation.
stationary arrangements “A Rotating Target Wheel for
4) Since any SHE production is a statistical process—only
Experiments with Superheavy-Element Isotopes at GSI
the average number of produced atoms in a given period is
Using Actinides as Target Material” (ARTESIA) has been
known, not the exact moment in time where a single atom
developed; see Figure 9. The gain arises from spreading out
is produced—many repetitions are inevitable for separathe beam over a larger target area thus reducing the beam
tions which operate discontinuously. This situation has led
power per surface area unit. Target material is electroto the construction of highly automated liquid-chemistry
deposited onto three 1.9 cm2 banana-shaped backing foils.[119]
set-ups.
Common to either arrangement—stationary or rotating
5) Even though in other fields, some techniques have
set-up—is a recoil chamber behind the target. Nuclear
reached the sensitivity required to observe or manipulate
reaction products recoiling out of the target are stopped in
single atoms or molecules, the observation of a characterhelium or another gas. Sufficiently volatile products (atoms or
istic nuclear-decay signature is presently the only means of
chemical compounds) are transported by the flowing gas
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Figure 8. Schematic diagram of a stationary target arrangement
together with the recoil chamber.
Figure 9. Photograph of the ARTESIA target wheel with three 248Cm
targets. The three long dark streaks indicate the area which was struck
by the first fraction of beam particles before the entire target area
(white surface) was “baked in” later.
through capillaries to the chemical- or detector-apparatus. For
nonvolatile products, usually aerosols (KCl or carbon “clusters”) are used as the carrier material in the gas (see ref. [116]
for more details). A schematic flow chart of the components
used in automated online chemical apparatus is depicted in
Figure 10.
378
Figure 10. Schematic flow chart of components for automated online
chemistry in the liquid phase (top) and in the gas-phase (bottom).
chemical behavior of the elements Rf through Sg in aqueous
solution. ARCA II allows fast, repetitive chromatographic
separations in miniaturized columns (8 mm long, 1.6 mm
internal diameter (i.d.)) with typical cycle times between 45
and 90 s. Depending on the chemistry, columns were filled
with cation- or anion-exchange resin or an organic extractant
on an inert support material. A photograph of the central
parts of the ARCA is shown in Figure 11. Common to all
batch-wise separations are time-consuming (ca. 20 s) evaporation steps (for sample preparation) that use IR light and hot
He gas. Separation times are typically between 5 s and 10 s.
A breakthrough in the automatization of the sample
preparation was achieved with the innovative “Automated
Ion exchange separation apparatus coupled with the Detection system for Alpha spectroscopy” (AIDA).[123–125] It has
recently been applied to detailed studies of Rf chemistry and
the first investigations on Db.[123, 125–128]
After batch-wise separations, individual samples are
assayed in detection systems for characteristic a-energies
and sf fragment energy measurements. To strengthen the
nuclide identification each event is logged together with time
information. This approach allows energy and time correlations between mother–daughter a–a or a–sf decay sequences
to be determined. To date, continuous liquid-phase separation
techniques have played a minor role in SHE chemistry (see
ref. [81, 93, 94, 114, 116] for more details).
5.2. Techniques and Instruments for Liquid-Phase Chemistry
5.3. Techniques and Instruments for Gas-Phase Adsorption
Chemistry
To date, almost all liquid-phase separations of transactinides were performed in discontinuous batch-wise operations with a large numbers of cyclic repetitions.[120] While in
several experiments on Rf and Db manual procedures were
used (see ref. [116, 121] and references therein for summaries
of the manual separation techniques) most transactinide
separations were carried out with automated instruments.
The implementation of the microcomputer controlled
Automated Rapid Chemistry Apparatus (ARCA)[122] yielded
the predominant share of today>s knowledge about the
Despite the fact that the transition metals in Groups 4–11
have very high melting points and that only a few inorganic
compounds exist, that are appreciably volatile at temperatures below about 1100 8C, gas-phase separations are important in the chemical investigations of SHE.[116, 129] Moreover,
as elements in Group 12–18 can presumably be employed
directly (in their atomic state) in gas-phase experiments, they
will play a major role in the chemistry of element 112 and
beyond. Since transactinide nuclei are usually stopped in gas,
a fast and efficient link can be established to a gas chromato-
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Figure 12. Basic principles of thermochromatography (left) and isothermal gas chromatography (right). The upper panels show temperature
profiles (l = column length) and the lower panels the deposition peak
in thermochromatography and the integral chromatogram in isothermal chromatography. From ref. [116].
Figure 11. Photograph of the computer-controlled ARCA. The central
part is the white block with two protruding magazines each carrying
20 chromatographic columns. The red cylinders are pneumatically
operated valves which route the solvent flow. The desired fractions are
sprayed from a glass capillary onto round Ta-discs seen on the
hotplate in the foreground and are then evaporated to dryness using
hot He from a ring-sized nozzle and a power controlled IR-lamp.
graphic system 1) by direct transport of volatile species in the
flowing gas, 2) by formation of a volatile compound in or at
the recoil chamber, and 3) by a transport with cluster
(aerosol) particles. As an additional advantage, gas-phase
separations can be operated continuously. Figure 12 shows
the basic principle of the isothermal chromatography as
compared with thermochromatography.
Early on, separations in the gas-phase played an important role in the investigations of transactinides. The technique
was pioneered by Zvara and co-workers at Dubna (see
refs. [72, 112, 130, 131] and references therein). In their experiments usually a thermochromatographic column was directly
connected to the recoil chamber. For more recent experiments, a coupling of the gas chromatographic columns to a
gas-jet transport system was developed.[132] Continuously
operating gas-phase separations were extremely instrumental
in studying the formation of halide and oxide compounds of
the transactinides Rf through Bh and to investigate their
characteristic retention time—a measure very often
expressed
as
a
“volatility”.
For
reviews
see
ref. [115, 116, 133]. Ref. [134] describes the online gas chromatographic apparatus (OLGA) and further setups are
explained in ref. [135–137]
The lower part of Figure 10 shows a flow Scheme for a
typical isothermal gas-chromatographic separation. Common
Angew. Chem. Int. Ed. 2006, 45, 368 – 401
to all of these experiments is the use of the known nuclide
half-life to determine a “retention-time-equivalent” to gas
chromatographic experiments.[78] On an atom-at-a-time scale,
it is the value for 50 % yield on a break-through curve
measured as a function of various isothermal temperatures.
The temperature corresponding to the 50 % yield at the exit
of the chromatography column is equal to the temperature at
which, in classical gas-chromatographic separations, the
retention time would be equal to the half-life of the
investigated nuclide. Products leaving the chromatography
column are usually attached to new aerosols in a so-called
“recluster process” and are transported in a gas-jet to a
detector system. There samples are assayed for time-correlated, characteristic a-decays and for sf fragments. Instead of
reclustering, a direct deposition of products leaving the
chromatographic column onto thin metal foils was used in
some seaborgium experiments[136, 137] and in an early experiment to search for element 107.[138]
A different experimental approach for gas-adsorption
studies is provided by thermochromatography.[113, 131, 139] In this
method, a (negative) temperature gradient is imposed on a
chromatography column. For the high-temperature version of
this method, ranging from about 450 8C to room temperature,
tracks from sf fragments are registered along the chromatography column after the end of the experiment.[140] While this
method is fast and highly efficient it has the disadvantages
that it>s temperature range is limited to about 450 8C by the
fission-track detectors and, more important, registration of sf
fragments alone is not nuclide specific.
Recently, low-temperature versions of thermochromatographic devices were developed and successfully applied in
the first chemical separation of hassium.[54] Their temperature
gradient ranges from ambient to liquid nitrogen temperature
(196 8C) and they are well adapted to investigate highly
volatile or gaseous species. A great advantage of these devices
named cryo thermochromatographic separator (CTS)[141]—
and its improved version cryo online detector (COLD)[54]—is
that the detectors form a chromatographic tube or channel.
This arrangement allows the detection of characteristic
nuclear decays with a high efficiency and high resolutions in
energy and the deposition temperature of an element or
compound at low temperatures. Individual cryo-detectors for
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condensation of highly volatile products on low-temperature
surfaces[142] were used in earlier searches for superheavy
elements.[143–145]
With (isothermal) gas-chromatographic experiments, volatile chemical compounds are usually formed by adding a
reactive gas in the hot entrance (reaction) zone ahead of the
chromatographic column. Compound formation can also be
carried out in the recoil chamber. Recent hassium experiments[54, 146] are examples for such an in situ volatilization in
which a reactive gas is a component in the transport gas.
Similarly, even in the very early thermochromatographic
experiments, volatile compounds were formed at the exit of
the recoil chamber and the appropriate techniques were
developed.[112, 140, 147, 148]
5.4. Perspectives of New Technological Developments
All breakthroughs in superheavy element chemistry were
linked to—and also in the future will be connected to—new
technical developments in experimental techniques and
apparatuses. More recently, a completely different kind of
coupling of a chemistry apparatus to the SHE production site
has attracted much attention and may become an important
tool in the future.[116] Coupling a kinematic recoil-separator—
the Berkeley gas-filled separator (BGS)—with the automated, fast centrifuge separation system SISAK has been
accomplished in a proof-of-principle experiment at the
Lawrence Berkeley National Laboratory (LBNL), Berkeley
(California). With this system 257Rf (t1/2 = 4 s) was separated
and identified in a continuous online liquid–liquid extraction.[93] Among other advantages, such a system completely
removes the primary heavy-ion beam from the reaction
products “beam” and it kinematically separates many
unwanted nuclides before they even enter the chemical
apparatus. The SISAK[93] experiment with its less specific
(limited energy resolution) but highly efficient and fast online
detection technique using an extractive scintillator,[114, 149]
greatly profited from the BGS as a preseparator. A miniaturized version dubbed SISAK III[150–152] is very well adapted
for studying short-lived nuclides with half-lives of the order of
1 s.
All experiments behind a recoil separator have the
advantage that a preseparated “beam” of a desired heavy
element becomes available. This approach may open up new
frontiers in direct chemical reactions with a large variety of
organic compounds,[153] and should allow gas chromatographic studies of superheavy element to be extended to a
much larger variety of compounds.
Another promising development is vacuum thermochromatography[154, 155] which has been used for lighter elements.[156, 157] This technique has the potential for very fast
separations in the millisecond region, possibly giving access to
short-lived nuclides of elements beyond Z = 114.
A not completely new but not fully exploited technique is
the so called three-column or multicolumn technique.[158–162]
To overcome difficulties with labor- and time-consuming
sample-preparation procedures typical for batch-wise experiments, this technique provides a different approach by
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continuously separating heavy elements in the liquid phase
and determining the chemical behavior of a transactinide
element from the ratio of long-lived daughter isotopes in one
or another fraction. First experiments have been performed
with Rf to study the fluoride complexation[158, 159, 161] and with
Db.[163, 164] Rf and Db, transported by the He(KCl) jet to the
chemistry apparatus, were continuously dissolved, and this
solution was passed through three consecutive ion-exchange
columns. Primary produced divalent and trivalent actinides
were “filtered” out on the first cation-exchange column. In
the next anion-exchange column anionic species were
retained for some time, while the following cation-exchange
column adsorbed cationic species—the long-lived a-decay
products of Rf. These were eluted after the end of irradiation
and were detected by offline a-spectroscopy.
One of the disadvantages of the multicolumn technique is
its limited range of half-lives and distribution coefficients. Its
big advantage is its potential to study short-lived isotopes with
half-lives of a few seconds. Because of its continuous
operation, it may allow these studies to be extended to
nuclides with cross sections well below the nanobarn level.
Preparations are under way to perform such studies[165] to
determine differences in the hydrolysis and complex formation of Mo, W, and Sg and to study the redox potential of SgVI.
6. Chemical Properties
Experimental results presented in this Section provide
important information on the chemical behavior of these
elusive elements. Discussing these properties in the context of
the properties of other elements, the structure of the Periodic
Table, or even the manifestation of relativistic effects is an
increasingly challenging task. It must be remembered that
with all the constraints in atom-at-a-time chemistry, only a
limited number of chemical properties can be studied
experimentally.
Formation of (a limited number of) chemical compounds
and volatilities of atoms and compounds were investigated
with thermochromatography and gas-chromatography
experiments by measuring adsorption temperatures and
retention times, respectively. The formation of complexes in
aqueous solutions, the behavior of these complexes, and their
interaction with a second phase (organic complexing solution
or ion-exchange resin) is studied in liquid-chromatography
and extraction experiments.
Results can only be compared with the behavior of other
elements investigated in the same experiment. Moreover, in
online gas-phase and thermochromatographic studies a direct
comparison is only meaningful if all the investigated nuclides
have about the same half-life.[131] This is because most shortlived nuclides decay before they reach, for example, their final
deposition temperature in a thermochromatographic experiment; at the end, products migrate very slowly along the
temperature gradient. Consequently, a seemingly too high
deposition temperature is determined; see Section 6.3.2. for
an example.
In the interpretation of experimental results, beyond the
pure analogy to the lighter homologues, assumptions are
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Superheavy Elements
made about the oxidation state or the type of compound
formed. Many important properties, such as ionic radii and
the stability of oxidation states, can only be judged indirectly,
for example, by comparison with the known properties of
lighter homologues in a group and their chemical behavior. In
addition, the chemical composition of SHE compounds is not
known and they are not accessible to classical structural
investigations. The compositions can only be assumed on the
basis of analogy in their experimental behavior. Empirical
model assumptions are always needed, for example, to
calculate physicochemical quantities such as adsorption
enthalpies[166] or sublimation enthalpies.[167] The step towards
the interpretation of these results in terms of relativistic
effects is an even more sophisticated task.
6.1. Rutherfordium (Rf, Element 104)
The Rf chemistry was pioneered by Zvara and co-workers
with experiments in the gas phase[72, 130] and by Silva et al. and
Hulet et al. in acidic, aqueous solutions.[73, 74] These experiments demonstrated that Rf behaves different from trivalent
actinides, and—as expected for a member of Group 4 of the
Periodic Table—Rf behaves similar to its lighter homologues
Zr and Hf. With the advent of a renewed interest in
transactinide chemistry in the late 1980s, many techniques
have been developed and used to extensively study Rf in
comparison with Group 4 elements and, in aqueous solution,
in comparison with tetravalent Th and tetravalent Pu ions as
Group 4 pseudo-homologues. These experimental results
have revealed a number of surprises but were not always
free of contradictory results between individual experiments,
and some were plagued by adsorption problems. Overviews of
Rf chemistry can be found in refs. [77, 79, 81, 90, 94, 109, 121].
Refs. [80, 120] concentrate on Rf properties in the aqueous
phase and refs. [115, 129, 130] on the behavior of Rf in the gas
phase.
6.1.1. Liquid-Phase Chemistry
Experiments in the aqueous phase concentrated on
unraveling the competing strength of hydrolysis and complex
formation with halide anions. In parallel, and to compare and
understand the measured distribution coefficients (Kd),
theoretical model calculations[108, 168] were performed to
compute hydrolysis constants and complex formation constants and described these processes for Group 4 elements
(M = Zr, Hf, Rf). The first hydrolysis step is described in the
reaction in Equation (3). At pH > 6 the pH-dependent stepwise hydrolysis (deprotonation) process gives rise to the
formation of M(OH)5 .
MðH2 OÞ8 4þ Ð MOHðH2 OÞ7 3þ þ Hþ
Ð MXðH2 OÞ7 3þ þ H2 O þ Hþ
...
Ð . . . MX3 ðH2 OÞ5 þ þ HX . . . Ð . . .
MX4 ðH2 OÞ4 þ HX . . . Ð . . .
ð4Þ
MX5 ðH2 OÞ þ HX . . .Ð MX6 2 þ H2 O þ Hþ
For hydrolyzed species it proceeds according to Equation (5).
MðOHÞ4 þ HX Ð MðOHÞX þ H2 O
...
Ð ...
ð5Þ
MðOHÞX3 þ HXÐ MX4 þ H2 O
Which process prevails and which are the most abundant
species in solution very much depends—apart from the kind
and concentration of the halide anion—on the pH value of
the solution. Among all halide complexes the ones with
fluoride ions are by far the most stable.
Fully relativistic molecular density-functional theory
(DFT) calculations of the electronic structures of hydrated
and hydrolyzed species and of fluoride and chloride complexes were used to compute free-energy changes for
hydrolysis and complex formation reactions.[168] For M4+
species, which undergo extensive hydrolysis at a pH > 0, it
was predicted that the hydrolysis decreases in the sequence
Zr > Hf > Rf.
Also the fluoride complex formation of non-hydrolyzed
species (present in strong acid solutions) decreases in the
sequence: Zr > Hf > Rf. However, it was realized that this
trend is inverted (Rf Hf > Zr) at a pH > 0 for the fluorination of hydrolyzed species or fluorocomplexes. Under these
less acidic conditions differences between the Group 4
elements are very small. Chloride complexation was calculated to be independent of pH value and always follows the
trend: Zr > Hf > Rf.
By combining all the results it was predicted that for a
separation—performed on a cation exchange resin in dilute
(< 102 m) HF—the Kd values will have the following trend in
Group 4: Zr Hf < Rf. This reflects the decreasing trend
Zr Hf > Rf in the formation of positively charged complexes.
Experimental results about the Rf behavior in comparison
with its lighter homologues (and pseudo-homologues) were
obtained from:
1) extracting neutral species into tributylphosphate (TBP)
with HCl and HBr solutions,[169–172]
2) extractions of anionic complexes with triisooctyl amine
(TiOA) with HF and HCl solutions,[173, 174]
3) ion-exchange studies of predominantly cationic species
with HF and HNO3 solutions,[175, 176]
4) ion-exchange studies of predominantly anionic species
with HF, HCl, and HNO3 solutions,[123, 127, 128, 175]
5) adsorption experiments on cobalt ferrocyanide.[177]
ð3Þ
The analogous step-wise complexation with halide anions
(X = F, Cl) proceeds for non-hydrolyzed species according to
Equation (4).
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MðH2 OÞ8 4þ þ HX
In all of these experiments Rf nuclear decay was directly
observed after the chemical separation procedure. While
some were procedures were manually performed batchextractions with separations of an aqueous and an organic
phase, the more recent ones were carried out as column
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chromatographic separations with the automated set-ups
ARCA II and AIDA (see Section 5.2). Investigations where
the behavior of cationic species of Rf (in dilute HF and mixed
HF/HNO3 solutions) were deduced from the observation of
long-lived nuclear-decay products have also been performed
with the multicolumn techniques (see Section 5.3).[158, 159, 161, 164]
These results are in agreement with ARCA and with AIDA
data.
The formation and behavior of neutral species were
characterized by extracting Zr, Hf, and Rf from 8 m HCl into
TBP. Column-chromatographic separations were performed
with a (undiluted) TBP coating on an inert support material.
The distribution coefficient of Rf was determined as 150(+64/
46) compared to a value of 53(+15/13) for Hf, obtained in
the same experiment.[171, 172] This result is in good agreement
with previously measured offline data (Kd(Hf) = 65, Kd(Zr) =
1180) and it gives the extraction sequence Zr > Rf > Hf
(Figure 13).
Figure 13. Distribution coefficients (Kd) for the extraction of neutral
species of Zr (~, c), Hf (*, g) and for Rf (&) at 8 m HCl/TBP.
Data from refs. [171, 172].
While this sequence seems to be somewhat surprising
based on empirical extrapolations, this sequence is expected
from the above mentioned theoretical considerations on the
competition between hydrolysis of the chloride complexes in
the aqueous solution and the formation and the extraction of
these complexes into the organic phase. The tendency for the
hydrolysis of Group 4 chloride complexes (the reverse
process of the complex formation) in 8 m HCl is then Hf >
Rf > Zr. Detailed discussions of earlier and partially conflicting results are given in ref. [79, 120]
To investigate cationic species, differences in the Zr, Hf,
Th, and Rf behavior in mixed 0.1m HNO3/HF solutions were
studied in cation-exchange-chromatography experiments
with ARCA.[175] Results are shown in Figure 14. For Zr and
Hf Kd values drop between 104 m and 102 m HF. For Rf this
decrease is observed at about one order of magnitude higher
HF concentrations, and it appears at even higher concentrations for Th. Therefore, the transition from cationic to
neutral and then anionic species requires higher HF concentrations for Rf than for Zr and Hf, but lower than the ones
needed for Th. This result establishes the following sequence
of F complex formation strength at low HF concentrations:
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Figure 14. Sorption of Zr, Hf, Th, and Rf on the cation-exchange resin
(CIX) Aminex A6 from 0.1 m HNO3 at various HF concentrations. As
indicated some data were obtained in offline (open symbols) and
some in online experiments. ? Rf online, * Hf online. Adapted from
ref. [94] with a revised version of data from ref. [175].
Zr Hf > Rf > Th.[175] For a similar system confirming data
were obtained with AIDA.[176]
These results are on a qualitative basis, that is, the
sequence of extraction and complex formation, in agreement
with theoretical expectations.[168] However, predicting
Kd values quantitatively still remains a challenging task for
theory, mainly because of the large variety of positively
charged complexes in solution and in extracted form. However, using the ionic radii[43, 178] of Zr (0.072 nm), Hf
(0.071 nm), Rf (0.078 nm), and Th (0.094 nm) it is appealing
to apply the hard soft acid base (HSAB) concept[179] in an
empirical approach to find an explanation of the observed
extraction sequence. In this concept it is assumed that the
hard F ion interacts stronger with small (hard) cations. From
this, what is expected, in agreement with the observation, is a
weaker F ion complexation of Rf than with Zr and Hf.
The experimental situation concerning the transition
towards anionic species at HF concentrations between
102 m and 1m HF remains somewhat ambiguous. In one
experimental series performed with an anion-exchange
resins,[175] for Zr and Hf the Kd values increase from about
10 to more than 100 between 103 m and 101m HF (measured
offline in batch-extraction experiments with long-lived tracers). This result is a continuation of the trend observed on
cation-exchange resin. For the Th offline data, and for the Hf
and Rf online data, no significant rise of the Kd values was
observed on anion-exchange resin for HF concentrations
between 103 m and 1m HF. While this is expected for Th,
which does not form fluoride complexes, it comes as surprise
for Hf and Rf. How much this experiment is affected by the
0.1m HNO3 in the solution remains unclear. Earlier experimental results suggest that Rf forms anionic F complexes in
pure 0.2 m HF, in mixed 0.27 m HF/0.1m HNO3, and
0.27 m HF/0.2 m HNO3 solutions.[158, 161] However, also these
experiments are not free of open questions. More experimental work is needed to confirm or reject these data and to
solve this puzzle.
The formation of anionic complexes from more-concentrated acid solutions is much more evident. Recent anionexchange chromatographic separations with AIDA showed
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that the adsorption of Rf (measured as percent adsorption)
increases steeply from 7.0 m to 11.5 m HCl (see
Figure 15).[123, 125] Typical for a Group 4 element, this behavior
Figure 15. Variation of the percent adsorption of Zr, Hf and Rf on the
anion exchange resin CA08Y from HCl at various concentrations. ? Rf
(Cm/Gd target), & Zr (Ge/Gd target), * Hf (Cm/Gd target), * Hf
(Ge/Gd target). Adapted from ref. [123].
goes in parallel with that of Zr and Hf, and is distinctively
different from that of the pseudo-homologue Th. The
adsorption sequence over the entire range is Rf > Zr > Hf.
This result can also be interpreted as the sequence in chloridecomplexing strength. However, this experimental outcome[123]
remains to be understood theoretically as it clearly contradicts earlier predictions.[168] First attempts to shed more light
on this question from experimental chemical-structure investigations using EXAFS spectroscopy are under way.[125]
Recent experiments with AIDA provide more exciting
and challenging data on the formation of anionic fluoride
complexes of Rf in comparison to its Group 4 members Zr
and Hf.[127] Measurements were made by anion-exchange
chromatography for 1.9 m to 13.9 m HF solutions. In this
concentration range it is important to realize that [HF2]
increases approximately like the “initial” concentration,
[HF]0, while the [F] remains almost constant. A decrease
of the Kd values of Zr and Hf with increasing [HF] is
explained as the displacement of the metal complex from the
binding sites of the resin by HF2ions. It is stunning to see
that, in contrast to the experimental results obtained in HCl
and HNO3 solutions, Rf behaves distinctly differently from Zr
and Hf. As shown in Figure 16, above 2 m HF the percent
adsorption for Rf on anion-exchange resin drops much earlier
and is significantly less than that of Zr and Hf up to 13.9 m HF.
A plot of Kd values versus the “initial” HF acid concentration, see Figure 17, also reveals a significant difference
between Rf and Zr and Hf. A slope of 2.0 0.3 of log Kd
against log [HF] was determined for Rf while the slope for Zr
and Hf is 3.0 0.1, indicating that different anionic fluoride
complexes are formed.[127] The slope analysis indicates that Rf
is present as the hexafluoride complex [RfF6]2—similar to
the well known [ZrF6]2 and [HfF6]2 at lower HF concentration—whereas Zr and Hf are presumably present in the
forms of [ZrF7]3 and [HfF7]3. The first measurement of a Rf
elution curve,[128] performed with 5.4 m HF on anion-exchange
columns, is in excellent agreement with previous data.
It was qualitatively discussed and suggested[125, 127] that
relativistic effects may strongly influence the fluoride-comAngew. Chem. Int. Ed. 2006, 45, 368 – 401
Figure 16. Variation of the percent adsorption of Rf, Hf, and Zr on the
anion-exchange resin CA08Y as a function of the initial HF concentration, obtained with the two different size columns: a) 1.6 J 7 mm
and b) 1.0 J 3.5 mm. ? 261Rf (Cm/Gd), * 169Hf (Cm/Gd), * 169Hf (Ge/
Gd), ! 167Hf (Eu), & 85Zr (Ge/Gd), ~ 89mZr (Y). Adapted from
ref. [127].
Figure 17. Variation of the distribution coefficient, Kd, of Rf, Zr and Hf
(as obtained with two different size chromatographic columns) on an
anion-exchange resin as a function of the “initial” HF concentration.
Rf (a), ? Rf (b), & Zr (a), & Zr (b), * Hf (a), * Hf (b);
a) 1.6 i.d. J 7.0 mm, b) 1.0 i.d. J 3.5 mm; i.d. = internal diameter.
Adapted from ref. [127].
plexing ability of Rf and, therefore, lead to the observed
differences. This possibility is deduced from relativistic DFT
calculations.[127] In this case, the trend in the orbital overlap
population between the valence d orbitals of M4+ and the
valence orbitals of F was found to be Zr Hf > Rf, suggesting that the Rf complex is less stable than those of Zr and Hf
for both the [MF6]2 and the [MF7]3 complex structures. This
result is different from the theoretically predicted sequence in
ref. [168] However, a quantitative theoretical understanding
still waits to be established.
A hypothetically Th-like or Pu-like behavior of Rf was
tested in AIDA with an anion-exchange resin and 8 m HNO3.
While Th and Pu form anionic complexes, and are consequently strongly adsorbed, Rf remains in solution[123] forming
cationic or neutral species—as expected for a typical Group 4
element with non-Th-like—and non-Pu-like properties.
6.1.2. Gas-Phase Adsorption Chemistry
The first[180] and the subsequent large number of pioneering experiments with Rf in the gas phase (see
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ref. [72, 130, 181, 182] and ref. [183] concerning the element>s
name) demonstrated that Rf—similar to its Group 4 homologue Hf—forms a chloride that is much more volatile than
the actinide chlorides. For some-time thereafter, the question
raised interest whether metallic (atomic) Rf behaves chemically like a typical member of Group 4 or whether it could
exhibit properties of a p-like element similar to Pb in
Group 14. This idea was triggered by a suggestion that
relativistically stabilized 7p1/2 orbitals could result in a
[Rn]5f147s27p2 ground-state configuration.[184] Support came
from (relativistic) multiconfiguration Dirac–Fock calculations
which resulted in a Rf ground-state configuration of
6d7s27p[185] or a mixing of 80 % 6d7s27p and 18 % 6d27s7p
(among other configurations).[186, 187]
Experiments searching for volatile atomic Rf—a typical
Pb-like behavior—did not show any p-like properties of Rf
and today this discussion is no longer relevant.[187, 188] This is
not only because of the now trusted 6d27s2 ground state (see
Table 2), predicted from a more recent and more accurate
coupled-cluster single-double (CCSD) excitations calculation
(see Table 2),[189] but also because there is sufficient confidence that a p-like ground state, which is only about
0.24 eV[186] or 0.5 eV[185]) below a d-like state, would not
results in typical Pb-like properties. Owing to energetically
favored formation of stronger bonds when forming compounds in the d-valence configuration, the low activation
energy is easily overcompensated. In addition, ionization
potential, atomic, and ionic radii for Rf are very similar to
those of Hf.
A new series of online gas-chromatographic studies were
performed in the 1990s to compare Rf with its lighter Group 4
homologues by using chlorinating[190] and brominating[191]
reagents (see ref. [115, 129] for reviews). In the chloride
system, theoretical considerations also excluded a Pb-like
behavior of Rf.[192] Besides the aim of determining the
formation and behavior of Rf compounds, the scope of
these experiments was to probe the influence of relativistic
effects on chemical properties.[193] This system seems to be
especially apt for obtaining a clear answer about the influence
of relativistic effects on a chemical property. From relativistic
calculations[107, 187, 194] RfCl4 was predicted to be more volatile
than HfCl4, whereas from non-relativistic calculations[107] and
from extrapolations of trends[195] within the Periodic Table
exactly the opposite behavior is expected. The results of these
different volatility predictions are shown in Figure 18 in terms
of vapor pressure versus temperature.
Because of the use of nuclides with very different halflives—a parameter which can strongly influence thermochromatographic results[113, 196]—it was almost impossible to precisely determine relative volatilities in the pioneering experiments. More recently, isothermal gas-chromatographic
experiments established that Rf chlorides are more volatile
than Hf chlorides (see left part of Figure 19).[190, 193, 197] This
feature has been interpreted as being the result of relativistic
effects. Unexpectedly, under similar experimental conditions,
Zr was observed together with Rf instead of showing a
behavior similar to Hf or an even lower volatility. This finding
remains puzzling. A study of Rf bromides showed the same
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Figure 18. Volatility in terms of vapor pressure as a function of
temperature for ZrCl4 and HfCl4 (experimental values) together with
theoretical predictions for RfCl4 including relativistic effects (rel) and
for a hypothetical non-relativistic (nr) case. Adapted from ref. [107].
Figure 19. “Break-through” yields for 261Rf (open symbols) and
165
Hf (closed symbols) tetrachlorides (left side) obtained from oxygenfree HCl as a reactive gas and for oxide chlorides (right side) formed
with SOCl2 vapor and oxygen as reactive gases. *, ^ 169Hf (t1/2 =
78.6 s); &, ~ 261Rf (t1/2 = 78(6/+11 s). Lines in the left part are results
from Monte-Carlo simulations. Adapted from ref. [193]. A activity,
T isothermal temperature.
sequence in volatility with Rf bromide being more volatile
than Hf bromide.[191, 197]
In Monte Carlo simulations of the chromatographic
process, the adsorption enthalpies (DH 0ðTÞ
) for single molea
cules on the quartz surface of the column are obtained by
finding a best fit to the experimental data by varying DHa as
the free parameter.[115, 196] Figure 20 shows a compilation[121] of
Group 4 element chloride and bromide adsorption enthalpies.
The experimental values for Rf show a striking reversal of the
(empirically) expected trend, which is however, in agreement
with relativistic theoretical model calculations.[107] Therefore,
this “reversal” is evidence for relativistic effects.
An estimate of the standard sublimation enthalpy (DH 0s)
can be obtained from a well established, empirical linear
correlation which exists between DH 0ðTÞ
and DH 0s for a
a
number of chlorides and other compounds (see ref. [130, 195]
and references therein). It is noteworthy that by using this
procedure a physicochemical quantity for macro-amounts can
be deduced from the behavior of a single atom.
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6.2.1. Liquid-Phase Chemistry
Figure 20. Adsorption enthalpies (DH0a) of chlorides and bromides of
Zr, Hf, and Rf on quartz surfaces. Adapted from ref. [121].
One problem when studying the pure Group 4 halides is
the possible formation of Group 4 oxy halide compounds. It
was shown in the chloride system that small amounts of
oxygen can lead to the formation of a less volatile oxy
chloride instead of the pure chloride. If present, the oxy halide
compounds may pose problems in the interpretation of
experimental results, especially if there are pronounced
differences in how easily Zr, Hf, and Rf form an oxy
chloride.[193] As seen in Figure 19, oxy halides are less volatile
than pure halides. Such a behavior was first observed in a
thermochromatographic experiment.[198] It is interesting to
note, that the behavior of RfOCl2 and HfOCl2 is much more
similar than the behavior of the pure halide compounds is.
This observation may be explained by the assumption that
oxy chlorides are only present in the adsorbed state and not in
the gas phase. The transport mechanism in Equation (6) was
proposed.[129]
MCl4ðgasÞ þ 1=2 O2 Ð MOCl2ðadsÞ þ Cl2ðgasÞ
ð6Þ
6.2. Dubnium (Db, Element 105)
A normal continuation in the Periodic Table puts
element 105, dubnium, Db, (see ref. [199] for element 105
names) into Group 5, below Nb and Ta. Early thermochromatographic separations of volatile chloride and bromide
compounds showed that Db behaves more like a transactinide
than an actinide element.[75, 112] These experiments also
indicated that Db chloride and bromide are less volatile
than the Nb halides.[75] In its first aqueous chemistry, Db was
adsorbed onto glass surfaces from HCl and HNO3 solutions, a
behavior very characteristic of Group 5 elements.[76] However, an attempt to extract Db fluoride complexes failed
under conditions in which extracts Ta complexes but not Nb
complexes. This observation provided evidence of unexpected Db properties,[76] and it triggered a number of followup investigations in aqueous solutions with ARCA which
revealed several, at-first-glance, unanticipated Db properties.
In the following Sections, illustrative examples of the Db
chemistry will be discussed. Overviews can be found in the
same references listed in Section 6.1 for Rf.
Angew. Chem. Int. Ed. 2006, 45, 368 – 401
The first detailed comparison between Db, its lighter
homologues Nb and Ta, and the pseudo-homologue Pa was
carried out with solutions at different HCl concentrations to
which small amounts of HF were added. Four series of liquid–
liquid extraction chromatography experiments were performed in ARCA II[122] with TiOA as a stationary phase on
an inert support.[200]
The first and second experiments with a total of 340 individual separations tested a typical behavior of the pentavalent
ions, namely the complete extraction of Nb, Ta, Pa, and Db
into TiOA from 12 m HCl/0.02 m HF and from 10 m HCl. As
expected, Db was found to be extracted together with Nb, Ta,
and Pa.
In the next series of 721 collection and separation cycles,
after the first extraction step, a Nb–Pa fraction was eluted
with 4 m HCl/0.02 m HF then a Ta fraction with 6 m HNO3/
0.0015 m HF. It came as a big surprise, that 88 % of the Db was
detected in the Nb–Pa fraction and only 12 % tailed into the
Ta fraction. This behavior is identical with that of Nb and Pa,
and distinctively different from that of Ta—a striking non-Talike behavior (under the given conditions).
To distinguish between a Nb-like and a Pa-like behavior,
in 536 experiments with 10 m HCl/0.025 m HF Pa was eluted
first, then came a Nb fraction with 6 m HNO3/0.0015 m HF. Db
showed an intermediate behavior (25 a events in the Pa
fraction and 27 a events in the Nb fraction) indicating that the
halide complexing strength of Db is in between that for Nb
and Pa. In a follow-up experiment using 0.5 m HCl/0.01m HF
to separate Pa and Nb, Db even showed more Pa-like
properties.[201] A summary of these results is shown in
Figure 21. These stunning results[200, 201] provided strong motivation to continue more detailed investigations of transactinides and laid the basis for a large experimental program.
The interpretation of these results was severely hampered
by the use of the mixed HCl/HF solution that did not allow
the complex formed to be clearly distinguished. In contrast to
the experimentally observed extraction sequence from HCl
solutions with small amounts of HF added, the inverse order
Pa @ Nb Db > Ta was theoretically predicted[202] for the
extraction from pure HCl solutions. This work considered the
competition between hydrolysis[203] and chloride-complex
formation. Recent experimental studies performed in the
pure F , Cl , and Br system[204] are in excellent agreement
with the theoretical predictions which include relativistic
effects.[202, 205] The fluoride complexation of Db in 0.2 m HF
was recently confirmed in an experiment which used three
consecutive ion-exchange columns—a cation exchange
(filter) column, an anion exchange (chromatography)
column, and another (filter) cation exchange column.[163] It
was shown that Db forms an anionic fluoride complex which
is strongly retained on the anion-exchange resin.
For the system Aliquat 336(Cl)—a quaternary ammonium salt which acts like a liquid anion-exchanger—and (pure)
6 m HCl, an extraction sequence of Pa > Nb Db > Ta was
determined (see Figure 22). This, in agreement with theoretical predictions,[202, 205] is the inverse to that in HCl solution
containing some HF. In series of offline and online experi-
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Figure 21. Fractional extracted activity of Ta (c), Nb (a), and
Pa (g) tracers as a function of HCl concentration in the system
TiOA-HCl/0.03 m HF. The bold bars encompass the upper and lower
limits deduced for the Db extraction from the elution position in
chromatography experiments at 10 m HCl/0.025 m HF, 4 m HCl/0.02 m
HF, 0.5 m HCl/0.01 m HF. The bar for the complete extraction of Db
from 12 m HCl/0.02 m HF is not included in the Figure for clarity. Data
from refs. [200, 201].EA = extracted activity.
Figure 22. Distribution coefficients for the extraction of Ta (*, c),
Nb (&, b), and Pa (~, a) tracers from pure HCl at various
concentrations into Aliquat 336(Cl). The Kd of Db at 6 m HCl (*) is
plotted with error bars encompassing 68 % confidence limits. Data
from ref. [204].
ments, Kd values of 1440, 683, 438 (+ 532/166), and 22 were
measured for the Pa, Nb, Db and Ta, respectively.
In pure HCl solutions, at concentrations above 2–4 m HCl,
all the elements Nb, Ta, Pa, and presumably Db, form the
same type of complexes, [M(OH)2Cl4]2, [MOCl4] ,
[MOCl5] , and [MCl6] (M = Nb, Ta, Db, Pa) with increasing
HCl concentrations. From the theoretically and experimentally determined strength of chloride-complex formation it is
observed that Pa forms a specific complex in a more dilute
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solution (starting with [Pa(OH)2Cl4] at 2–4 m HCl) followed
by the other elements at higher concentrations. The complexes of Nb start to form next ([Nb(OH)2Cl4] above 4–5 m
HCl), while the [Ta(OH)2Cl4] complex is formed at or above
6 m HCl. This suggests that at 6 m HCl the complex
[Db(OH)2Cl4] is predominant. Additional experiments[204]
confirmed the theoretically expected sequence of complexing
strength among the halide anions: fluoride @ chloride > bromide. More information on other, previously performed
experiments is given in refs. [77, 79–81, 109, 120, 121]
Once well-developed and “tested”, fast chemical-separation procedures for Db were at hand—which used the
detection of nuclear decay properties of 262Db to characterize
the Db chemistry. These techniques were applied to obtain
important information on the nuclear properties.[206] With
100 mL of 0.05 m a-hydroxyisobutyric acid, Db fractions were
eluted within 6.5 s from a cation exchange resin in ARCA and
were prepared for a-spectroscopy. The chemically separated
samples had a purity that allowed the successful search for,
and discovery of, the new isotope 263Db (t1/2 = 27 s) and its
decay properties to be determined.[206, 207]
6.2.2. Gas-Phase Adsorption Chemistry
All detailed investigations to determine the volatility of
Db and Group 5 element compounds in the gas-phase have
been performed with pentahalides (chlorides and bromides)
and with the significantly less-volatile oxy halides. Owing to
the high tendency of Group 5 elements to react with trace
amounts of oxygen or water vapor to form oxy halides,[131, 208, 209] investigations of pure halides require an intensive purification of all gases and a very careful preconditioning of the quartz chromatography columns with the halogenating reactive carrier gas prior to each experiment. Under
certain conditions it was even necessary to add an additional
reactive component, for example, BBr3 was added to the Br2,
to prevent the formation of an oxy bromide.[131, 148] Some of
the literature data may have suffered from—and their
interpretation may have been obscured by—the unintentional
formation of oxy halide compounds. When searching for
differences in volatilities of the pure halides of Nb, Ta, and
Db, differences in the formation probability of oxy halides
along the Group 5 elements may add an additional complication, especially in experiments at the limits of feasibility.
For the lighter Db homologues in group 5 the following trends
have been observed:
1) The volatility of MX5 decreases in the sequence F > Cl >
Br > I, which can be compared with Cl > Br > I > F in
Group 4.
2) All oxy halides are less volatile than the corresponding
halides, which is also true in Group 4.
3) Group 5 halides are less volatile than the corresponding
ones in Group 4.
A discussion of these properties from a theoretical point
of view can be found in refs. [43, 44, 107].
The early thermochromatographic experiments in chlorinating and brominating atmospheres had already qualitatively shown that the Db chlorides and bromides are less
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volatile than the Nb compounds.[75] However, half-life corrections had to be applied which made a quantitative
comparison questionable. More recently, the lower volatility
(compared to the lighter homologues Nb and Ta) of a Db
bromide compound was confirmed in a gas-chromatographic
experiment with OLGA.[210] Interestingly, the volatile Ta
bromide was only formed when BBr3 was added to the HBr.
A volatility trend of Nb Ta > Db was observed. This
sequence is very surprising since theoretical calculations,
which included relativistic effects, predict a higher volatility
for the DbBr5 that is presumably formed than for NbBr5 and
TaBr5 ;[107, 211] similarly to the observed sequence in Group 4. It
was speculated,[115] that instead of the pentabromide the
DbOBr3 may have been formed and investigated in these
experiments. However, this implies that under identical
conditions Nb and Ta form pentabromides while Db forms
an oxy bromide. This situation would be in agreement with
theoretical calculations[212] showing a stronger tendency for
Db to form oxy halides than Nb and Ta and could be viewed
as an influence of relativistic effects in Db. A different
theoretical approach showed a monotonic trend in the
stability of monooxides for the series Nb, Ta, and Db.[213]
Several attempts to form the pure Db pentachloride
failed. The results of the last series of experiments with
OLGA, which used elaborated purification techniques, are
shown in Figure 23.[115] Conditions were used which allowed a
Figure 23. Relative yield (yrel) of Db (~) measured in online gaschromatographic experiments with purified HCl. The Nb results were
obtained under oxygen-free conditions (*, pO2 = 1 ppm) and with
oxygen present (^, pO2 100 ppm). Curves are best fit Monte Carlo
simulations with the adsorption enthalpies (DHa) as a fit parameter.
Adapted from ref. [115].
well established break-through curve for NbCl5 to be
measured in preparatory experiments. However, Db showed
a behavior, which was interpreted as the result of the presence
of two species, namely DbCl5 and DbOCl3. Compared to
NbOCl3, DbOCl3 became “volatile” at an approximately
50 8C higher temperature—indicating a lower volatility for
DbOCl3 than for NbOCl3. For what was interpreted as a
DbCl5 component, only a volatility limit was established. The
final comparison on the halide volatility of Group 5 elements
including Db remains to be performed.
Angew. Chem. Int. Ed. 2006, 45, 368 – 401
6.3. Seaborgium (Sg, Element 106)
In 1974, A. Ghiorso, J. M. Nitschke and co-workers
discovered element 106 in the reaction 249Cf(18O,4n)263Sg.[214]
For 20 years, 263Sg (t1/2 = 0.9 s), which is produced at a rate of
about six atoms per hour, was the longest-lived known
isotope. Its short half-life and a low production rate prohibited chemical investigations for a long time. While the adecay properties of 263Sg, which were observed in the
discovery experiment, were confirmed in investigations of
271
Ds (its a-decay chain passes through 263Sg),[49, 57] none of
these experiments supported evidence for the previously
reported sf decay in 263Sg.[215]
With the large number of detailed chemical investigations
of Rf and Db came the development of more sensitive, new
and improved experimental techniques. Together with the
firm belief—based upon nuclear-reaction systematics and
calculations—that it would be possible the produce a longerlived, hitherto unknown isotope of Sg in the 22Ne + 248Cm
reaction, preparations started in the first half of the 1990s to
perform chemical investigations of Sg in the aqueous phase
and in the gas phase. A large international collaboration
finally involving 16 institutes from nine countries crystallized
around nuclear-chemistry groups at the GSI, Darmstadt, the
University of Mainz, and the University Bern–PSI, Villigen.
Shortly before the first Sg experiment was performed at GSI,
a Dubna/Livermore collaboration reported the discovery of
265,266
Sg with half-lives in the ten-second range in the
envisioned reaction.[216, 217] The door to detailed chemical
investigations of Sg was pushed open.
Seaborgium is expected to behave chemically like a
member of Group 6 with Cr, Mo and W as its lighter
homologues. Oxides, oxy halides, and hydroxy halides are
important and characteristic compounds of these elements.
The formation and the properties of such Sg compounds in
comparison with lighter homologues have been investigated
in the aqueous phase and in the gas phase.
6.3.1. Liquid-Phase Chemistry
The first chemical separation and characterization of Sg in
aqueous solution[218, 219] was conducted using 265,266Sg produced
by irradiation of a 248Cm target with 22Ne projectiles from the
GSI UNILAC. The nuclear reaction products were transported to ARCA II,[122] dissolved in 0.1m HNO3/5 O 104 m HF,
and were separated on a cation-exchange resin. To probe the
Sg behavior in comparison with its lighter Group 6 homologues Mo and W, and to distinguish it from a U-like behavior,
3900 identical separations were performed with a cycle/
repetition time of 45 s. The “Sg-fraction” was always the
first 10 s of elution. A typical chromatogram obtained with W
tracer is shown in Figure 24.
Three correlated a–a decays of 261Rf and 257No—daughter
products of 265Sg—were detected. From these three observed
atoms it was concluded[218, 219] 1) that Sg elutes together with
Mo and W, 2) that it behaves like a typical Group 6 element
and forms hexavalent ions, 3) that, like its homologues Mo
and W, Sg forms neutral or anionic oxide or oxy halide
compounds, and 4) that it does not form seaborgyl ions
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Figure 24. Elution curve for W-tracer (Me6+), modeling the Sg separation on ARCA in 0.1 m HNO3/5x104 m HF, together with “lower limit”
(lower right arrow) for the elution of di- , tri-, tetravalent ions, and
UO22+. Elutions were performed at room temperature with a flow rate
of 1 mL min1 from 1.6 J 8 mm columns filled with the 17.5 2 mm
particle size cation-exchange resin Aminex A6. Adapted from ref. [219].
([SgO2]2+) making it different from its pseudo-homologue U
which remains as [UO2]2+ on the cation exchange column. By
analogy with its Mo and W homologues, it can be assumed
that under the given conditions Sg forms a (hydrated) anionic
complex such as [SgO3F] or more likely [SgO2F3] or the
neutral species SgO2F2. The result of the first experiment was
that Sg exhibits properties very characteristic of Group 6
elements, and does not show U-like properties.
Owing to the low fluoride concentration in the first
experiment, the seaborgate ion ([SgO4]2) could not entirely
be excluded. To check this option and the influence of the
fluoride ion, a second series of experiments was performed
with pure 0.1m HNO3 as a mobile phase.[220] Contrary to the
lighter homologues Mo and W, Sg was not eluted from the
cation-exchange resin in the absence of HF. From this it is
concluded that F ions significantly contributed to the
complex formation in the first experiment. This observation
rules out that Sg was eluted as [SgO4]2 in the first experiment. The non-tungsten-like behavior of Sg in pure HNO3
may be attributed to the weaker tendency of Sg6+ to hydrolyze.[220, 221] While Mo and W can reach the neutral species
MO2(OH)2 (M = Mo, W) for Sg hydrolysis presumably stops
at
[Sg(OH)5(H2O)]+
(sometimes
characterized
as
[SgO(OH)3]+) or even at [Sg(OH)4(H2O)2]2+; these are
species which presumably remain adsorbed on the cation
exchange resin. This trend in the hydrolysis of metal cations is
not only theoretically predicted for Group 6 (Mo > W > Sg)
but also for Group 5 (Nb > Ta > Db); see refs. [43, 44] for a
compilation of theoretically predicted and experimentally
observed hydrolysis sequences.
6.3.2. Gas-Phase Adsorption Chemistry
As a member of Group 6, Sg is expected—in analogy to its
lighter homologues Cr, Mo and W—to be very refractory
(that is, high melting) in the elemental state but to form
volatile halide, oxy halide, oxy hydroxide and carbonyl
compounds.[222] Although only a very limited number of
relatively unstable hexahalides exist for Group 6 elements, a
much larger variety of more or less stable oxy halides of the
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types MOX4 and MO2X2 (M = Mo, W; X = F, Cl) are known.
They are volatile enough for gas-phase separations between
400 8C and 150 8C. Based on these properties several experiments were performed to probe the Sg behavior in comparison with its lighter homologues. A slightly different kind of
experiment was used to investigate the moderately volatile (at
temperatures at and above 1000 8C) oxides (MO3) and slightly
more volatile oxy hydroxides (MO2(OH)2). A discussion of
relative stabilities and volatilities of these Group 6 compounds can be found in refs. [129, 195, 223] from an experimental and empirical point of view and in
refs. [43, 194, 224, 225] based upon relativistic theory—including aspects of the stability of the predominant hexavalent
oxidation state in Sg.
At the time when the Sg experiments at GSI were under
preparation, the nuclear-chemistry group at Dubna did first
experiments with what was assumed to be the sf isotope 263Sg
(t1/2 = 0.9 s) produced in a 249Cf(18O,4n) reaction.[140, 226, 227] As a
chemically reactive component, 20 % air saturated with
SOCl2 vapor was added to the Ar carrier gas to form volatile
chlorides or oxy chlorides. A thermochromatographic technique with fission track detectors was applied (see Section 5.3
for advantages and disadvantages of this technique).
Although in two experiments with a quartz wool filter in
the start section of the chromatography column no events
were observed, in four subsequent experiments without any
filter a total of 41 sf tracks were registered along the
column.[140] These events peaked at a temperature of approximately 270 8C (and tailed below 100 8C) while the 176W tracer
(t1/2 = 2.5 h) peaked at a much lower temperature of around
80 8C. The following arguments were given as evidence that
the observed tracks are from the decay of 263Sg and are not
any “background” from nuclides with Z < 106 (Db, Rf,
actinides), sputtered target material, or U impurities in the
column: [140] 1) A distinct chromatographic peak was observed
which could be described by a theoretical curve, 2) a background run with the column at ambient temperature and no
reactive gas added gave only 18 sf tracks located directly at
the column entrance, and 3) further nuclear decay and cross
section arguments supported the 263Sg decay.
Based on these results, the following compounds and
scenarios were postulated to describe the observed distributions: Volatile WO2Cl2 is formed quickly and deposits in its
proper (high) temperature range; the compound then reacts
in a few dozens of seconds to yield the more volatile WOCl4
which is found in the lower temperature part of the column.
This interpretation was supported by the experimental
observation[227] that short-lived Mo and W isotopes are
found at higher temperatures compared to the lower temperature deposition region for longer-lived 176W. By analogy, it
was proposed that SgO2Cl2 was formed and deposited at
about 270 8C where it decayed before the more volatile
SgOCl4 could be formed and transported. Therefore, no
information on similarities or differences in the Sg behavior
compared with its lighter homologues can be deduced from
the experimental observation. Additional criticism against
this experiment mainly came from the open question whether
the observed sf tracks really originated from the decay of
263
Sg.[81, 109]
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In 1995, a large international collaboration started with
online isothermal gas-adsorption chromatography experiments at the GSI.[218] In this work, the longer-lived isotopes
265
Sg (t1/2 = 7.4 s) and 265Sg (t1/2 = 21 s; for nuclear properties
see ref. [97])—produced in the reaction of 22Ne with 248Cm—
were used to study the formation and volatility of oxy
chlorides with the OLGA set-up.[134] The reaction zone was
kept at 1000 8C and it was connected to a chromatography
column made of fused silica. Reactive gases—Cl2 saturated
with SOCl2 and traces of O2—were introduced to form oxy
chlorides. Break-through curves were measured for Mo, W,
and Sg compounds[218, 228] in the temperature range between
about 150 8C and 400 8C by varying the temperature of the
isothermal chromatography column from experiment to
experiment. All the products leaving the column were
transported to a detector set-up for an unambiguous identification of Sg by its characteristic a-decay chains.[97] Results of
the break-through measurements which reflect the volatility
are shown in Figure 25. Under these conditions, the formation
Figure 25. Relative yield (yrel) of MO2Cl2 (M = Mo, W, and Sg) breaking
through the column as a function of the isothermal temperature in the
column. ^ 85MoO2Cl2 (t1/2 = 58.8 s), a DHa = 90 3 kJ mol1,
168
&
WoO2Cl2 (t1/2 = 51 s), g DHa = 96 1 kJ mol1, ! 265SgO2Cl2
(t1/2 = 7.4 s), c DHa = 985/ + 2 kJ mol1. Error bars are given
with a 68 % confidence limit. They are asymmetric for errors on small
numbers.[229] Adapted from ref. [228].
of MO2Cl2 compounds (M = Mo, W, Sg) is most likely. The
sequence in volatility of MoO2Cl2 > WO2Cl2 SgO2Cl2 was
established. Monte Carlo simulations of these experimental
results gave adsorption enthalpies of DHa(MoO2Cl2) = 90 3 kJ mol1,
DHa(WO2Cl2) = 96 1 kJ mol1,
DHa
(SgO2Cl2) = 98 (+ 2/5) kJ mol1—the first thermochemical
properties of Sg to be determined.[228] Thus, it was shown that
Sg forms oxy chlorides analogous to those of Mo and W, while
U forms the different type of molecule, UCl6, which has a
much higher volatility. This Sg behavior is in line with
extrapolations in Group 6 and with relativistic theory calculations.[225] A detailed discussion of the thermochemical
characterization of Sg is given in ref. [223].
The adsorption enthalpies were measured with trace
amounts (for W) or one-atom-at-a-time (for Sg) at zero
surface coverage. Well established, empirical linear correlations[195, 223] between adsorption enthalpies (obtained at zero
coverage)—measured for a large number of chlorides and oxy
Angew. Chem. Int. Ed. 2006, 45, 368 – 401
chlorides on quartz glass—and sublimation enthalpies allow a
macroscopic quantity for Sg to be derived from only a few
investigated
atoms:
DH 0s(SgO2Cl2) =
127(+10/
1 [228]
21) kJ mol .
Based on this quantity it is expected that
the Sg metal has an equally high or even higher sublimation
enthalpy than W and, therefore, Sg is one of the least volatile
elements or even the least volatile element in the Periodic
Table.[129, 228]
Owing to the strong tendency of Group 6 elements to
form oxides, and to the experimental difficulties to avoid even
traces of oxygen, to date, no experimental attempts have been
made to study pure halides. However, studies of the formation
and of the volatility of oxy hydroxide compounds of Sg were
developed[136, 230] and were performed in gas-adsorption chromatographic experiments.[137] To cope with the lower volatility
of the oxides and oxy hydroxides these studies were carried
out in a high-temperature gas-chromatography apparatus.[136]
The reaction zone—a quartz wool plug in the entrance section
of the quartz column—was kept at about 1050 8C. Here, O2
gas, saturated with H2O at 50 8C, was added as a reactive gas.
The subsequent main part of the quartz chromatography
column (about 40 cm) was held at an isothermal temperature
of around 1000 8C. Products leaving the column were
collected on 25 mm thin, cooled aluminum foils which were
rotated in front of detectors to assay these samples for
characteristic decays of Sg and W. While these temperatures
are challenging for gas-chromatographic studies, the oxy
hydroxide system profits from highly efficient separations not
only of actinides but also Rf and Db because the oxide
compounds of these elements have very low volatilities.
From the results of preparatory experiments[230] with Mo
and W it was expected that for Sg the transport mechanism of
the oxy hydroxide compounds would also not be a simple
reversible adsorption–desorption but would rather occur
through the dissociative adsorption and associative desorption process in Equation (7), sometimes also called “reaction
gas-chromatography”:[230]
MO2 ðOHÞ2ðgasÞ Ð MO3ðadsÞ þ H2 OðgasÞ
ð7Þ
Even at high temperatures, retention times of these
processes are generally longer than simple adsorption–
desorption processes. From the behavior of short-lived
166,168
W tracer a typical retention time of 8 s was calculated.
Therefore, the longer-lived isotope 266Sg (t1/2 = 21 s) was used
in the experiment to investigate oxy hydroxide properties,
despite it having a lower cross section than the usually used
265
Sg (t1/2 = 7.4 s). From the observation of only two Sg atoms
(two 266Sg a-decays shortly followed by the sf of the 262Rf
daughter) passing through the column, it was shown that Sg
forms a volatile oxy hydroxide SgO2(OH)2, a property typical
for Group 6 elements.[137]
6.4. Bohrium (Bh, Element 107)
The fourth transactinide element Bohrium is a member of
Group 7 and, therefore, is expected to exhibit properties
similar to its lighter homologues Mn, Tc, and Re. The same
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result is obtained from fully relativistic DFT calculations of
the electronic structure of the Group 7 oxy chlorides MO3Cl
(M = Tc, Re, Bh).[231] Of these compounds, BhO3Cl is the
most covalent and relatively stable. Increasing values of
dipole moments and of electric-dipole polarizabilities within
Group 7 are reasons for the following theoretically expected
sequence of volatilities: TcO3Cl > ReO3Cl > BhO3Cl.[231] The
same order results from classical extrapolations of thermochemical properties down Group 7 which also predict BhO3Cl
to be more stable and volatile than TcO3Cl and ReO3Cl.[232] If
only trace amounts or single atoms are present it can safely be
assumed that the species MO3Cl is formed with oxidizing
chlorinating gases. The oxy halide compounds of Group 7
elements reflect their intermediate position between the
lighter transactinides, which form volatile halides, and the
highly volatile Group 8 tetroxides.
In addition to the oxy chlorides, also oxides and oxy
hydroxides of Group 7 elements are relatively stable and
volatile compounds. Early attempts to chemically investigate
BhO3(OH) were based on these properties.[138, 233] These
experiments, performed with 249Bk and the even more
precious 254Es as target material, were not successful because
of an insufficient detection sensitivity. However, test experiments with Re have shown that the oxy hydroxide system has
some potential for future studies of Bh, especially if volatile
Po, Pb, and Bi contaminations, which mask the a-spectra and
limit the sensitivity, can be avoided by using a physical
preseparator.[234–236]
To date, the only information on chemical properties of
Bh was obtained in gas chromatographic experiments[99] on
the oxy chlorides which were performed by Eichler and coworkers at the PSI Philips cyclotron in Villigen. Extensive
tests with Tc and Re tracer activities had been performed
previously.[232] The experimental set-up for the Bh experiment
was similar to the one used in the previous Sg experiments
(see Figure 26).
Figure 26. Schematic view of the Bh gas-chromatography experiment.
Adapted from ref. [99] original drawing courtesy of R. Eichler.
The isotope 267Bh (t1/2 = 17 s), which had just been
discovered in an experiment at the 88-inch cyclotron at
LBNL,[98] was produced in the 249Bk(22Ne,4n) reaction. All
reaction products were carried with C-aerosols as a cluster
material from the recoil chamber to the OLGA. The reactive
gases HCl and O2 were added in front of the high-temperature
zone of the reaction oven which was kept at 1000 8C. In the
oven, C-aerosols were burned and the oxy chloride compounds were formed. Relative yields of the compounds were
measured as a function of the isothermal temperature in the
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quartz chromatography column. With the aid of CsCl as a
recluster material the compounds which left the chromatography column were transported to the detection system.
A total of six genetically linked 267Bh decay chains were
observed;[99] four at an isothermal temperature of 180 8C, two
at 150 8C, and none at 75 8C. The results are shown in
Figure 27 as relative yields (the four events at 180 8C were
Figure 27. Relative yields (yrel) of the compounds 108TcO3Cl (*),
169
ReO3Cl (*) and (most likely) 267BhO3Cl (&) as a function of the
isothermal temperature (T). The error bars indicate a 68 % confidence
interval. Solid lines are from Monte Carlo simulations with the
standard adsorption enthalpies of 51 kJ mol1 for TcO3Cl,
61 kJ mol1 for ReO3Cl, and 75 kJ mol1 for BhO3Cl. The dashed
lines are calculated relative yields based on the 68 % confidence
interval of the standard adsorption enthalpies of BhO3Cl from 66 to
81 kJ mol1. Adapted from ref. [99].
normalized to the required 22Ne-beam intensity and were
taken as the 100 % value) together with results from tracer
experiments and with Monte Carlo simulations using a
microscopic model[196] to determine standard adsorption
enthalpies. The characteristic 50 % yield of the BhO3Cl
curve is located at a higher temperature than the one for
TcO3Cl and ReO3Cl.
Qualitatively, this result shows that Bh behaves like a
member of Group 7 and forms a volatile oxy chloride—
presumably BhO3Cl—which is less volatile than the chloride
compounds of the lighter homologues.[99] The sequence of
volatility is: TcO3Cl > ReO3Cl > BhO3Cl. More quantitatively, the deduced BhO3Cl adsorption enthalpy of DH 0a =
(75 + 9/6) kJ mol1[99] is in excellent agreement with a
theoretical prediction which includes relativistic effects.[231]
This result coincides with the value expected from empirical
correlations of thermochemical properties assuming Bh is in
Group 7.[232] As with the Sg oxy chloride, an empirical
correlation was used for BhO3Cl to estimate the sublimation
enthalpy: DH 0s(BhO3Cl) = 89 + 21/18 kJ mol1 in comparison with DH 0s(ReO3Cl) = 66 12 kJ mol1 and DH 0s(TcO3Cl) = 49 12 kJ mol1.
6.5. Hassium (Hs, Element 108)
Element 108 was discovered by MInzenberg and coworkers at the GSI in 1984 in the reaction 208Pb(58Fe,1n)265Hs.[237] With a half-life of 1.5 ms for 265Hs no
chemical studies of Hs seemed to be possible until the more
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neutron-rich isotope 269Hs (t1/214 s) was observed in 1996 in
the discovery of element 112[53] as a member of the 277112 adecay chain. But the observed rate of about one atom per
week, although not unexpected, was discouragingly small.
However, it was expected that the direct production of 269Hs
in the 248Cm(26Mg,5n) hot-fusion reaction could be accomplished with production rate of about a factor of ten higher.[96]
The existence of a t1/2 14 s isotope[48] (t1/2 9 s was obtained
from the first observation[53]) and the prospect for a detection
of about one atom per day provided enough faith to prepare
the first chemistry experiment. Of course, novel techniques
for irradiation, separation, and detection had to be developed
and deployed to reach the required sensitivity. The first
successful Hs chemistry experiment was conducted again in
the framework of a large international collaboration at GSI>s
UNILAC in 2001.[54] It did not only yield the first chemical
information on Hs but also provided new and interesting
nuclear results.[100] Among these were evidence for the newly
found isotope 270Hs, the first nuclide with the N = 162
neutron-shell, and a confirmation of the element 112 discovery by measuring concordant decay properties in a-decay
chains starting from chemically separated element 108 fractions. A compilation of earlier, interesting but less successful
attempts to perform Hs chemistry experiments can be found
in ref. [129]
The heavier elements in Group 8, Ru and Os, show a
unique property among all transition metals—they exploit the
highest possible oxidation state (8 + ) and form highly volatile
tetroxides MO4 (M = Ru, Os). It was therefore most attractive
to investigate HsO4 as the first chemical compound of Hs. The
experimental set-up used in the first Hs chemistry experiment
is schematically shown in Figure 28. This experiment was
unique, in a number of aspects, and different from recent gaschromatographic experiments:
1) A rotating target wheel (“ARTESIA” in Figure 28) for the
precious and highly radioactive 248Cm targets[119]—in
combination with a gas-jet transport system—was applied
for the first time in SHE chemistry. This set up enabled
higher beam intensities to be accepted and, consequently,
provided larger production rates.
2) The chemical reaction with the reactive gas O2 was
performed “in situ” in the recoil chamber named in situ
volatilization and online detection (IVO).[54, 238] An oven
attached to the recoil chamber provided a fast and
efficient oxidation of stopped recoils. A similar technique
has previously been used for lighter elements in combination with high-temperature thermochromatography.[112, 140, 147, 148] In the Hs experiment this approach
allowed highly volatile compounds to be transported
without any cluster material in a very dry He/O2 gas
mixture over 10 m in a teflon capillary to the detection
system.
3) The cryo online thermochromatography separator and
detector (COLD)[54, 239] was mainly used and as an alternative its forerunner CTS could also be employed.[141]
COLD consists of 36 pairs of silicon PIN-photodiodes
(with silicon nitride surfaces) coupled to a support which
provides a negative temperature gradient between about
20 8C at the inlet and about 170 8C at the exit.
Nuclear decay chains originating from the known isotope
Hs and from the isotope 270Hs, for which first evidence was
obtained in the course of this experiment, were observed[100]
in a narrow peak[54, 239] along the temperature gradient; see
Figure 29 for observed nuclear decay chains and Figure 30 for
their distribution along the temperature gradient.
From the observation of seven molecules of HsO4 and
their adsorption position at 44 6 8C (comparison with
OsO4 : 82 7 8C), it is concluded that Hs forms a relatively
stable, volatile tetroxide—as expected for a typical member of
Group 8.[240, 241] However, the exact adsorption position is at a
surprisingly high temperature,[54] that is, HsO4 exhibits an
unexpected low volatility[242] or, in other terms, has a high,
negative adsorption enthalpy. From a best fit of Monte Carlo
simulations (solid line in Figure 30) to the experimental data
the following adsorption enthalpies on silicon nitride were
deduced:[54] DH 0ðTÞ
(HsO4) = 46 2 kJ mol1 for HsO4 and
a
0ðTÞ
DH a (OsO4) = 39 1 kJ mol1 for OsO4.
For the first time, this Hs-chemistry experiment showed
that the 1 pb cross-section limit can be reached in SHE
chemistry—a crucial prerequisite to explore the chemistry of
SHE in the region around element 114.
The second Hs experiment[146] was performed by a
collaboration using a set-up dubbed continuously working
arrangement for clusterless transport of in situ produced
volatile oxides (CALLISTO).[243] Again 269,270Hs were produced in the 26Mg on 248Cm reaction. A small amount of
enriched 152Gd was added to one 248Cm target segment to
simultaneously produce a-decaying 172Os (t1/2 = 19 s) and
173
Os (t1/2 = 22 s). These isotopes were used to monitor the
Os behavior under identical conditions.
In the CALLISTO experiment, as in the
preceding experiment, tetroxides were
formed in a recoil chamber and in its
hot (600 8C) outlet section. Contrary to
the preceding experiment, which had
used dry gases, in this experiment water
was added (2 g H2O per kg gas) to the O2/
He mixture. These gases transported
volatile products over a distance of
about 13 m in polytetrafluroethylene
(PTFE) capillaries within 3–4 s to a set
of four detector boxes. Each detector box
Figure 28. Schematic view of the low-temperature thermochromatography experiment used to
contained a linear array of four detectors
investigate HsO4. Adapted from ref. [54] original drawing courtesy of Ch. D,llmann.
Angew. Chem. Int. Ed. 2006, 45, 368 – 401
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M. Schdel
Figure 29. Nuclear decays chains from Hs isotopes observed in the first chemistry experiment of element 108. Indicated are the registered
energies for a-particles and sf-fragments and the lifetimes. Adapted from ref. [54, 100].
Figure 31. Distribution of the deposited amount of 172,173OsO4 and
269,270
Hs (presumably as HsO4). For Hs, the positions of the six
correlated decay chains are depicted. Data from ref. [146].
Figure 30. Experimentally observed thermochromatogram of HsO4
(filled histogram with arrow at detector 3) and of OsO4 (open histogram with arrow at detecto 6) given in relative yields. Solid lines
represent results of a Monte Carlo simulation of the migration process
of 269HsO4 and 172OsO4 along the temperature gradient assuming
standard adsorption enthalpies of 46.0 kJ mol1 and 39.0 kJ mol1,
respectively. Adapted from ref. [54, 239].
(PIN-diodes) facing at a distance of 1 mm a stainless steel
plate coated with a thin film of NaOH. Computer controlled,
three detector boxes in a row, that is, 12 detectors, were
always measuring while the fourth box was refurbished and
freshly prepared NaOH was mounted. The water in the
transport gas maintained the chemical reactivity of the NaOH
layer over the measuring period.
The adsorption of the osmate (viii) along the chemically
reactive NaOH surface is shown in Figure 31 in terms of
detector position. More than 50 % of the Os is found on
detector 1 and the rest of the detectors exhibit a significant
tailing. Six decay chains of Hs were detected in the first five
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detectors (one a–a–a correlation and five a–sf correlations).
They were centered at detector 3 (see Figure 31).[146] The low
statistics of the six Hs events does not allow any conclusion to
be drawn about a possible lower reactivity of the HsO4 as
compared to OsO4. However, the observation confirms the
formation and stability of the volatile HsO4 compound, and
shows the similarity in chemical reactivity between HsO4 and
OsO4. Presumably, the deposition of Hs is the result of the
formation of a hassate(viii) according to Equation (8).
2 NaOH þ HsO4 ! Na2 ½HsO4 ðOHÞ2 ð8Þ
For the first time, an acid–base reaction was performed
with the tetroxide of Hs.[146, 243]
6.6. Elements 109–111
To date, no attempts have been made to investigate
chemical properties of SHE located in Groups 9–11: meitnerium (Mt, element 109), darmstadtium (Ds, element 110), and
roentgenium (Rg, element 111). Low production rates in
combination with very short half-lives (t1/2 = 0.1 s) for isotopes
produced in cold-fusion reactions provide an insurmountable
hurdle for chemical investigations. However, 48Ca-induced
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Superheavy Elements
nuclear reactions with actinide targets (see Section 6.7) may
provide access to longer lived nuclides of these elements
which will allow chemical studies. Early on, a large number of
preparatory test experiments were performed, mainly as
thermochromatographic studies, with lighter homologues
radiotracers.[244–247] Based on the work in ref. [248] these
tests showed that volatile oxides or hydroxides may be good
species for a chemical separation and characterization of Mt
and Ds. In Group 11, the presumably rather volatile compound RgCl3 could be compared with AuCl3.[248]
Theoretically, the elements in Group 11 are extremely
well studied; see for example, the comprehensive Review by
PyykkS about gold.[249] Highly advanced relativistic coupledcluster calculations[250] yield for the Rg atom a ground state of
6d97s2, which is in contrast to its lighter homologues. Very
large relativistic effects are observed on orbital energies and
level sequence for the Rg atom[250] and for the RgH
molecule.[251, 252] These calculations suggest that the chemistry
of Rg will be largely dominated by relativistic effects and,
consequently, Rg will behave more like a typical d-block
element in contrast to its lighter homologues Ag and Au. The
importance of the spin-orbit coupling on the bonding and on
molecular properties of Au and Rg hydrides, halides, and
oxides was pointed out in ref. [253].
6.7. Element 112
For a large number of reasons, element 112 is one of the
most exciting elements for nuclear chemist, from a nuclear
and a chemical perspective. While in typical cold-fusion
reactions, such as in the discovery of element 112,[53] isotopes
with half-lives of milliseconds and microseconds are produced, the hot (or warm) fusion reaction 238U(48Ca,3n)
seemed to provide access to the isotope 283112 with a longer
half-life. First experiments had observed a sf activity with a
half-life of about 5 min produced with a cross section of about
4 pb.[71] New information on the decay properties of 283112
indicated that this nuclide may preferentially decay by aemission with a half-life of about 4 s to the t1/2 = 0.2 s daughter
nuclide 279Ds which decays by sf.[66] Either decay mode would
allow chemical investigations of element 112. Noted, however, that these findings are yet unconfirmed. Confirmation
experiments for 283112 at the LBNL have remained unsuccessful[254] and a new attempt is being made at the GSI. Also
chemistry experiments on element 112 will be able to confirm
or disprove these claims.
Even though the first scientific articles are entitled
“Chemical Identification and Properties of Element 112”[68, 69]
and “Chemical and Nuclear Studies of Hassium and Element 112”[70] the reported results can only be taken as first
evidence for a (possible) chemical behavior of element 112.
Therefore, an improved experimental program is under way.
A large international collaboration has performed two
experiments at the GSI—so far with no conclusive
results[255]—and is continuing. Nevertheless, the techniques
applied and the first results reported in refs. [68–70, 255] are
so exciting that reporting them herein is justified (though with
the appropriate restraint).
Angew. Chem. Int. Ed. 2006, 45, 368 – 401
Very early on, the question how closely element 112
would resemble the chemistry of Hg—its lighter homologue
in Group 12—attracted a lot of attention and was summarized
by Fricke.[35] Based on relativistic calculations which show a
strong stabilization of the closed 7s2 shell, Pitzer[40] indicated
the possibility that element 112 is relatively inert—almost like
a noble gas—and, in elementary form, would be a gas or a
very volatile liquid. Therefore, element 112 should be more
volatile than Hg.
A very recent fully relativistic treatment of the interaction
of element 112 with metallic surfaces, such as Au and Pd,
predicts weaker adsorption of 112 than Hg on these metals.[256]
These quantitative, most advanced calculations predict that
the adsorption temperature of element 112 on (ideal) Au
surfaces will be 93 8C below that for Hg. In addition it is
pointed out that element 112 will form a (weak) metal–metal
bond (if the Au surface is sufficiently clean) and, therefore,
element 112 will adsorb at much higher temperatures than Rn
which is adsorbed only by van der Waals forces. Adsorption
enthalpies of element 112 on metal surfaces obtained from an
empirical model also indicate a weak chemical bond formed
on Au surfaces (no bond is formed on an Fe surface) and a
“volatile noble metal” character of element 112 was predicted.[167, 195, 257] The volatility of element 112 was expected to
be much higher than that of Hg.
Note, however, that despite its relative inertness, element 112 may have a rich chemistry involving complex
formation in (sufficiently oxidizing) aqueous solutions and
in the gas phase.[35, 252] Theoretically observed[258] dissimilarities between Hg and element 112, for example, different
ground-state configurations of the monocations (d10s1 for
Hg+, d9s2 for 112+), large differences in the level sequence of
Hg2+ and element 1122+, strong level mixing in element 112,
can be taken as evidence for interesting differences in the
chemical behavior of these two remarkable elements.
The first two experimental attempts to chemically separate and identify the t1/2 = 3 min sf-isotope 283112 have been
made by Yakushev and co-workers at the FLNR in
Dubna.[67–69] Products from the 48Ca + 238U reaction—including Hg produced through a small amount of Nd added to the
U target—were transported in the elementary state in a He
gas flow. In the improved, second experiment the 25 m long
capillary with a 2 mm internal diameter was connected to a
detector arrangement of eight pairs of gold-coated passivated
implanted planar silicon (PIPS) detectors (each with a surface
area of 3.25 cm2 at ambient temperature) for the detection of
the mercury-like element 112. After this apparatus came a
5000 cm3 cylindrical ionization chamber for the detection of a
gaseous element 112. This chamber had an aerosol filter at the
entrance. In addition, a mixture of Ar and CH4 was added as a
detector gas. The PIPS detectors and the ionization chamber
were placed inside a barrel-shaped neutron detector with
126 3He-counters.[68]
In this experiment, the online produced and measured Hg
was found, as expected, on the Au-coated PIPS detectors
(95 % of the t1/2 = 49 s 185Hg was adsorbed on the first pair at
an He-flow rate of 500 mL min1, this rose to 99 % at
250 mL min1). At the end of the experiment eight sf-decays
had been registered in the ionization chamber and none in the
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M. Schdel
PIPS detectors. With an expected background of only about
one event and based on arguments that exclude any other sfsource originating from the nuclear reaction these sf-events
were attributed to the decay of 283112.[68] It was concluded that
element 112 does not form a strong metal–metal bond with
Au and that it behaves more like the noble gas Rn rather than
Hg. An estimate for the upper limit of the adsorption
enthalpy for element 112 (on Au)—based on the nonobservation of element 112 on the Au surface—gave DHa
(112) 60 kJ mol1.[68] This value can be compared with
DHa(Hg) 100 kJ mol1
and
DHa(Rn) = 29 3 kJ mol1.[155] However, as excellent as it was, this experiment
still leaves some questions open and it does not fully establish
the proof that, what was observed, reflects properties of
element 112. It should be taken as evidence and it calls for
more and more definite, quantitative experiments.
A second series of experiments started in the year 2003 at
the GSI in a large international collaboration with ten
institutes from five countries.[70] Again, the reaction of 48Ca
with 238U was selected to produce 283112. Simultaneously,
220
Rn is also produced as a transfer product from U as well as
184186
Hg from small amounts of Nd in the target. These
experiments are aiming at measuring the adsorption behavior
of element 112 on Au in comparison with that of Hg and Rn.
Along the Au surface, a temperature gradient from + 35 8C to
about 185 8C is established in a modified version of the
COLD detector, which was so successful in the Hs experiment.[54] Test experiments determined the adsorption behavior of Hg and Rn on various transition-metal surfaces (with
some interference owing to an ice coverage at the lowtemperature end).[259] A schematic view of this set up is shown
in Figure 32. In the first GSI experiment on element 112[70] the
In the first experiment at the GSI the following was
observed:[70]
1) As expected, even at room temperature Hg is mainly
adsorbed ( 50 %) on the Au (opposite the first detector)
and its exhibits a tail which is in agreement with a Monte
Carlo simulation of the adsorption process assuming an
adsorption enthalpy of DHa(Hg) = 101 kJ mol1.
2) From the adsorption behavior of Rn, which begins at
detector 29 and peaks around detector 31, and from the
measured resolution of the a-spectra, it was concluded
that below 95 8C the Au surface was covered with a thin
ice layer.
3) At an expected background of three events, five sf-events
were observed which scatter along the entire detector
array.
4) A cluster of seven events—very cautiously interpreted as
possible candidates for the sf decay of the t1/2 3 min
283
112—were observed at detectors 29 to 31.
Again, this result seems to suggest that element 112
behaves like a gaseous metal. However, owing to some small
imperfections and open questions, the international collaboration agreed to first repeat this experiment under improved
conditions to substantiate the findings of their first experiment. The second experiment—performed at the GSI in the
fall of 2004—worked with an improved set up and was
sensitive to shorter lived nuclides.[255] However, there was
already indication from the first, preliminary data analysis
that also this experiment did not yield a final, conclusive
result.[255] More experiments are needed to shed light on the
chemistry of element 112.
7. Summary and Perspectives
Figure 32. Schematic view of the experimental set up used in the first
experiment on element 112 at the GSI.[70] Figure is courtesy of S.
Soverna.
Au-catcher was facing an array of 32 silicon PIN-diodes, in the
second experiment an improved version was developed and
applied which allows measuring much more efficiently in
(almost) 4p-geometry.[255] Further improvements made the
second experiment also much more sensitive to shorter halflives in the region of a few seconds.
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As already observed in the pioneering experiments, more
recent results from manifold studies again justify positioning
of the transactinides—or superheavy elements (SHE)—,
beginning with element 104, into the seventh period of the
Periodic Table (see Figure 2). To date chemical studies on
their behavior in the aqueous phase have been performed
with Rf, Db, and Sg. Studies in the gas phase have been
carried out for Rf, Db, Sg, Bh, and Hs, and have now reached
element 112.
Up to element 108, all experimental results yield properties which, in general, place these elements into their
respective group of the Periodic Table, that is, Rf, Db, Sg,
Bh, and Hs into Groups 4, 5, 6, 7, and 8, respectively. This
result demonstrates that the Periodic Table still remains an
appropriate ordering scheme also regarding the chemical
properties of these elements. However, a closer and more
subtle look reveals that all the more detailed chemical
properties of these elements—in comparison with their
lighter homologues—are no longer reliably predictable by
simple extrapolations in the Periodic Table. Even if for an
element such as Sg for which, from an agreement of the
observed chemical properties with empirical observations, an
“oddly ordinary”[260] behavior is found, and if it looks as if Sg
is “back on track”[261] this cannot be taken as evidence that
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Superheavy Elements
also the heavier elements will behave “as expected”. Sometimes relativistic effects and other effects, such as shell effects,
may just cancel to “mimic” a normal behavior. However,
modern relativistic atomic and molecular calculations, in
combination with empirical models, allow for quantitative, or
semi-quantitative, comparisons of experimental and theoretical results, and they show excellent agreement in a number of
cases. From this situation it can be deduced that relativistic
effects strongly influence the chemical properties of the
transactinides. This is expected to become even more distinct
when proceeding to even heavier elements.
Fascinating and challenging prospects are ahead to
embark from the known territories of the element 106 for
aqueous chemistry and the element 108 for gas-phase
chemistry, to explore the chemical “terra incognita”. The
region between element 108 and 114 is waiting to be “chemically” discovered. A door to more neutron-rich, longer lived
isotopes was opened by the use of the doubly magic nuclei
48
Ca as a beam and neutron-rich actinides as target materials
(see Section 1 for details and for references). If we assume for
the moment that yet unconfirmed results on the existence of
relatively longer lived nuclides of spherical SHE around Z =
114 can be substantiated, then this opens the possibility for
many chemical studies on all the SHE up to at least
element 114. For heavier elements, even more challenging
experiments are ahead as a result of the decreasing cross
sections and shorter half-lives.
Prospects to study chemical properties of SHE beyond
element 106 in the aqueous phase mainly depend on the
development of methods to cope with production rates of less
than one atom per hour and a wide range of half-lives.
Existing separation techniques, such as ARCA and AIDA,
will remain essential tools to shed more light on the diverse
and often unexpected behavior of the lightest SHE. Moreover, this region continues to serve as an excellent test ground
for detailed predictions of chemical properties with the most
advanced theoretical model calculations.
Well developed gas-phase chemistry techniques are at
hand to deepen our insights into many unresolved questions
of compound formation, volatility, and adsorption behavior of
Group 4–8 halides, oxides, and mixed compounds. With the
advent of new techniques, such as the coupling to physical
recoil separators, volatile, organometallic compounds may be
accessible too. Most challenging and most fascinating will be
the upcoming gas-phase studies of metallic transactinides
beyond Group 8. Because of their relativistically stabilized
inert, closed-shell electrons (7s2 and 7s27p1/22), which put
elements 112 and 114 into a unique position among the SHE,
chemical studies of these elements have the highest priority
for the near future.
Coupling of chemical separation set ups to physical recoil
separators will provide a big leap in the quality of separation
and detection of SHE. The first successful steps in this
direction were made at the BGS in Berkeley. Not only
continuously operating liquid–liquid extraction devices, such
as the already tested SISAK, but also gas-chromatographic
set ups and emerging vacuum-thermochromatography techniques will greatly profit from such a coupled system. In
addition to fascinating chemistry aspects, these experiments
Angew. Chem. Int. Ed. 2006, 45, 368 – 401
will certainly provide multifaceted nuclear data and they are
vital tools for a clear identification of the atomic number of
SHE.
Abbreviations and Acronyms
Acronym
Full name
Aliquat 336 Methyltrioctylammonium chloride
AIDA
Automated Ion exchange separation apparatus coupled with
the Detection system for Alpha
spectroscopy
ARCA
Automated Rapid Chemistry
Apparatus
ARTESIA A Rotating Target Wheel for
Experiments with SuperheavyElement Isotopes at GSI Using
Actinides as Target Material
BGS
Berkeley Gas-filled Separator (at
the LBNL)
CALLISTO Continuously Working Arrangement For CLusterLess Transport
of In-Situ Produced Volatile
Oxides
COLD
Cryo On-Line Detector
CTS
Cryo Thermochromatographic
Separator
FLNR
Flerov Laboratory of Nuclear
Reactions, Dubna, Russia
GSI
Gesellschaft fIr Schwerionenforschung, Darmstadt, Germany
DH 0ðTÞ
Adsorption enthalpy
a
DH 0s
Standard sublimation enthalpy
IUPAC
International Union of Pure and
Applied Chemistry
IUPAP
International Union of Pure and
Applied Physics
IVO
In situ Volatilization and Online
Detection
JWP
Joint Working Party (of IUPAC
and IUPAP)
Distribution coefficient
Kd
LBNL
Lawrence Berkeley National
Laboratory, Berkeley, California
OLGA
Online Gas chromatographic
Apparatus
RIKEN
The Institute of Physical and
Chemical Research, Wako, Japan
QED
Quantum electrodynamics
sf
Spontaneous fission
SHIP
Separator for Heavy Ion Reaction Products (at the GSI)
SHE
Superheavy element(s)
SISAK
An automated, fast centrifuge
separation system for continuous
liquid-liquid extraction studies
SO
Spin-orbit
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Section[a]
6.2.1
5.2
5.2
5.1
5.4
6.5
5.3
5.3
1
1
6.1.2
6.1.2
1
1
6.5
1
6.1.1
5.4
5.3
1
4.2
2.2
1
1
5.4
4.2
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t1/2
TBP
TiOA
TWG
UNILAC
M. Schdel
Half-life (in nuclear decay)
Tributylphosphate
Triisooctyl amine
Transfermium Working Group
(of IUPAC and IUPAP)
Universal Linear Accelerator (at
the GSI)
1
6.1.1
6.1.1
1
1
[a] Section in which the abbreviation or acronym is first
mentioned.
I thank my group members at the GSI and my fellow
colleagues, especially R. Eichler, H. W. Gggeler, K. E. Gregorich, J. V. Kratz, Y. Nagame, N. Trautmann, and A. Trler
for many years of very fruitful collaborations. V. Pershina is a
continuous help to me with her profound theoretical knowledge and S. Hofmann with his expertise about the synthesis and
decay of the heaviest elements. S. Soverna and Ch. Dllmann
provided me with artistic drawings of experimental set ups. I
acknowledge the help of B. Schausten for her work on the
graphics, the time W. Brchle spent reading and correcting my
first draft, and, especially, G. Herrmann?s and E. K. Hulet?s
suggestions to improve the manuscript. I thank G. Mnzenberg
for his continuous support for nuclear chemistry. Last not least,
it was G. Herrmann who laid the foundations for the superheavy element chemistry, not only at the GSI.
Received: June 24, 2004
Revised: May 11, 2005
Published online: December 19, 2005
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